The mitogenic responsiveness of spleen cells obtained from avian sarcoma virus-inoculated Fischer 344 rats was studied. Sixty % of the rats had astrocytomas, 13% had sarcomas, 7% had mixed gliosarcomas, and 20% had no evidence of tumors. Only spleen cells from rats bearing astrocytomas had significantly diminished responses to phytohemagglutinin and concanavalin A (Con A) when compared to control responses. The decreased responsiveness observed with phytohemagglutinin was limited to the optimal concentration range (10 and 20 microgram) while a broader concentration of Con A (0.01 to 50 microgram) induced significant suppression. Moreover, a more profound immunosuppression was observed with Con A. The results also demonstrated that spleen cells from rats with the largest astrocytomas exhibited the greatest suppression. From the results of this study, it appears the avian sarcoma virus-induced astrocytoma in rats is an immunological parallel of the human disease based on the loss of general immunological competence as assessed by responsiveness of lymphocytes to phytohemagglutinin and Con A.