An in vitro study was carried out to determine the susceptibility of pig lymphocytes to infection by 4 strains of pseudorabies virus (PRV) and the response of the cells to 3 different mitogens. The PRV strains used were USDA, 13-1, 3-1 and Norden, while the mitogens were phytohemagglutinin (PHA) Concanavalin (Con A) and lipopolysaccharide S (LPS). The susceptibility of lymphocytes was evaluated by accessing the viral end point dilution, at which cytopathogenic effects (CPE) were achieved, when infected pig lymphocytes were inoculated onto Vero cells monolayer. Lymphocytes supported replication of the following viruses: USDA, and Norden strains, while 3-1 and 13-1 replications were not supported. This probably indicates that 3-1 and 13-1, which are TK negative strains of pseudorabies, were either unable to absorb onto pig lymphocytes or did absorb and penetrated but were quickly destroyed by the lymphocytes. The CPE observed when USDA and Norden infects L phi and, in certain circumstances, may destroy the lymphocytes. The significance of these findings is discussed below.