Twenty-five pigs inoculated with African Swine fever virus were studied for the development of changes in lymphocyte numbers and function. The results indicated that lymphopenia and decreased percentage of circulating T-lymphocytes occurred as early as 7 days after the pigs were inoculated. These were accompanied by depressed lymphocyte function, as measured by mitogen- or antigen-induced blastogenesis. There were proportionately greater decreases in T-lymphocyte numbers and function than were found for B-lymphocytes. Studies of phagocytic cell function revealed decreased phagocytic function for peripheral blood monocytes and unchanged (normal) neutrophil function. Depressed T-lymphocyte function occurred in pigs with acute and subacute infections, indicating depression of cellular immune function.