We investigated the proliferative response to mitogens of spleen mononuclear (Spm) cells from Cryptococcus neoformans-infected rats. We determined reactive oxygen intermediates (ROI) and nitric oxide (NO) production by peritoneal and Spm cells, and evaluated the correlation of the proliferative response with NO and ROI production. The proliferative response of Spm cells from infected rats dramatically decreased at 14 and 21 days postinfection (PI). The unresponsiveness of Spm cells from 14-day infected rats was not abrogated by the addition of L-NAME and AG, indicating that NO is not involved in the antiproliferative response of experimental cells. When SOD, catalase, and indomethacin were added to the cultures, the suppression was still observed, indicating that ROI and prostaglandins are not involved in the unresponsiveness of lymphocytes. The proliferative response of lymphocytes from 14-day infected rats was significantly improved when cultures were made in the presence of Con A and exogenous IL-2. Additionally, a purified T-rich fraction from infected rats cultured with control macrophages recovered the normal proliferative response. This result indicates that macrophages from infected rats mediate the unresponsiveness of lymphocytes, probably by reducing the ability of lymphocytes to secrete IL-2.