A panel of monoclonal antibodies have been produced which recognize monomorphic determinants of sheep MHC Class II antigens, including an allogenically derived murine monoclonal antibody specific for the I-E gene product. Immunoprecipitation and SDS-PAGE analyses indicates that these monoclonal antibodies recognize a non-covalently associated glycoprotein complex of molecular weight 30-32 kDa (alpha chain) and 24-26 kDa (beta chain). One and two colour immunofluorescence was used to measure the distribution of these 'Ia-like' antigens on mononuclear cells from various lymphoid organs. They were found almost exclusively on lymphocytes expressing surface immunoglobulin (B lymphocytes) and on a small population of surface immunoglobulin negative cells. Most thymocytes were negative for Class II molecules while thymic epithelial cells were positive. The tissue distribution of Class II molecules was found to be similar to that described in man. Individual monoclonal antibodies displayed no variations in reactivity with the different tissues studied.