Humoral (plaque-forming) and cellular (mitogenic) responses were monitored as a function of time after infection with a threshold lethal dose of Rauscher virus in virus-susceptible SJL/J mice and virus-resistant C57B1/mice. Plaque forming response was suppressed in the SJL/J, whereas in the C57B1/10, it was slightly elevated during the 2nd and 3rd weeks after infection; this correlated with a mild viremia. Increasing the dose of virus 40X did not significantly alter the humoral response pattern of either strain and did not cause the C57B1/10 response to mimic that of the SJL/J. For both strains the cellular immune response pattern differed from the humoral. Depression of T-cell mitogen responses occurred in the SJL/J, but at a later time than humoral response suppression. Neither T- nor B-cell mitogen responses were affected in the C57B1/10. The effect on mitogen responsiveness appeared to be independent of that on plaque-formation in both strains. © 1978 Pergamon Press,Inc.