Despite the increasing knowledge of the MHC in many animals, little is known about the MHC of the horse. Little is known about the function of the equid immune system with most studies placing emphasis on strictly immunogenetic analysis of the equine MHC. At the beginning of these studies very little was known about the class II MHC antigens of the horse or their role in immune function. Therefore investigations of the MHC were undertaken in horses with particular emphasis on studies of the class II MHC antigens. It was possible to utilise the mixed lymphocyte reaction (MLR) to reveal polymorphism in the DR-like antigens of the horse. The MLR technique was used to determine the DR like antigen expression among members of a family group. There was significant polymorphism among the DR antigens of the horse, with between 5 and 9 alleles, in the equine populations studied. Secondary stimulation was shown to occur between animals that were negative in the primary MLR, indicating the existence of a secondary group of stimulatory alloantigens, probably DP-like. The autologous mixed lymphocyte reaction (AMLR) was surprisingly high, equine lymphocytes undergoing significant proliferation when cultured with autologous lymphoid cells. A buoyant adherent macrophage/monocyte cell provided the most efficient stimulation of both the alloreactive (MLR) and autologous (AMLR) proliferative responses. After plastic adherence these cells were the most effective stimulators of both the AMLR and the MLR, as lymphocyte proliferation in AMLR matched that in the MLR. An equine equivalent of the human DR antigens appeared responsible for stimulating the proliferation in the MLR as antibody to DR caused significant suppression of the MLR. The AMLR was not suppressed by this antibody. The response to the mitogen concanavalin A (Con A) was used to test immune function in the horse. Lymphocytes from many horses showed no response to Con A while others underwent significant proliferation. Lymphocytes taken from responder animals were able to suppress the response to Con A after exposure to Con A. If these cells were cultured without added Con A for three days and then pulsed with Con A they underwent greater proliferation than uncultured PBL subject to a primary exposure to Con A. Lymphocytes cultured in Con A were not able to suppress the MLR. Attempts to produce antibodies in the horse by injection of allogeneic lymphocytes, to class II MHC proved exceedingly difficult, although antibodies to possible new local ELA specificities ELA-AQ*12, -AQ*l3 and BQ*2 (possible class I) were investigated. There was linkage between the genes controlling the cellularly defined DR specificities and the class I specificities investigated, although there was a recombinant between the class I loci and the D locus. Attempts to produce antibodies to class II antigens prepared by electrophoresis in both rabbits and mice proved unsuccessful. In addition three antibodies to human DR antigens proved to be useful in these studies. These antibodies were used to determine the expression of class II on equine lymphoid cell populations. All lymphoid cell populations examined expressed significant quantities of class II antigens with monocytes and B cells expressing significantly more than T cells (1.5 - 2x). However, the T cells still expressed significant quantities of class II in the horse confirming the observations of others (Crepaldi et al., 1986; Monos et al., 1989). Not surprisingly the cells which acted as the best alloantigen-presenting cells expressed the most class II. Addition of Con A to an equine lymphocyte culture increased cell class II expression regardless of whether or not the cells proliferated in response to Con A, indicating an active suppressor mechanism initiated by Con A binding to equine lymphocytes. Immunoprecipitation experiments demonstrated that the equine DR-like antigens have a similar sub-unit structure to human DR with a heavy (α) and a light chain (β) with MW of 32 KO and 28 KO respectively. Urea-isoelectric focussing revealed two main groups of protein bands, one group of bands in the basic range (β chain) and another in the acidic range (α) chain. The equine DR β-chain appeared to be polymorphic while the DR α -chain displayed little polymorphism. The technical difficulties associated with the use of cellular or protein electrophoretic techniques led to the use of molecular biological techniques to define the equine MHC. Human DRB, DQA cDNA probes were used to demonstrate polymorphism in the equine class IT genes. After determining the inheritance of the band patterns revealed by each probe it was possible to show that the DRB gene is closely linked to the DR gene product that controls the MLR, with no recombinants among 15 matings. RFLP analysis using a human cDNA clone to the DQA gene indicated extreme polymorphism for equine DQA. The DQA gene is linked to the DRB gene as five horses inherited the parental genotype and there was a single recombinant between the putative DQA and the DRB loci. It was also possible to demonstrate that the genes controlling the MLR were more likely to be linked to the DR than the DQ genes because the recombinant was also between the D and the DQA genes. Preliminary experiments demonstrated the existence of a polymorphic DPB gene which tends to confirm the earlier observation that a DP like antigen, responsible for alloreactivity between animals negative in the primary MLR, is expressed on equine lymphocytes. Adherent cells had increased levels of DRBl RNA when compared with non-adherent cells, confirming the protein data.