Observations of aflatoxin induced thymic involution were followed by studies of other parameters of cellular immunity. Guinea pigs treated with 0.03 to 0.35 mg aflatoxin B1 for 3 to 5 weeks were tested for the development of cutaneous hypersensitivity, migration inhibition factor activity, and stimulation of lymphocyte transformation. A significant suppression of cutaneous hypersensitivity to a Nocardia asteroides sensitin in sensitized aflatoxin recipients was demonstrated. In vitro reductions in migration inhibition factor (MIF) and depressed stimulation indices (Phytohemagglutinin) of peripheral blood lymphocytes were also demonstrated in aflatoxin recipients but did not reach significant levels.