Six pigs were infected oro-nasally with a moderately virulent African swine fever (ASF) virus from the Dominican Republic (DR II). The effect of virus infection on the pig's immune system was tested by measuring peripheral leucocyte numbers and the ability of mononuclear leucocytes (MNL) to respond by lymphocyte proliferation (LP) to the mitogens phytohemagglutinin-P (PHA-P), concanavalin-A (Con-A), and pokeweed mitogen (PWM). All 6 pigs developed high viremias between 4 and 18 days post-inoculation (DPI) which became undetectable by 32 to 46 DPI. Virus was found in erythrocytes, plasma, and mononuclear leucocytes from peripheral blood. Overall, virus infection had only minor effects on the number of circulating leucocytes, lymphocytes, monocytes and granulocytes. At the early acute phase of infection slight neutrophilia and lymphocytopenia were observed with mildly elevated monocyte numbers and slightly depressed neutrophil numbers that continued from the time of evident reduction in viremia to beyond the period of viral clearance. The infected pigs readily produced high titers of ASF virus antibody shortly after the onset of viremia. No significant differences in LP responses of MNL from the 6 pigs to PHA-P, Con-A and PWM were observed after infection when compared to those obtained with MNL from normal pigs. The in vitro addition of infectious ASF virus to MNL from normal pigs did not affect LP responses to any of the three mitogens. These results do not support the hypothesis that immunosuppression is a consequence of ASFV infection of pigs.