In vitro proliferative responses of T lymphocytes in the peritoneal exudate cells of C3H/HeN(Iak) mice immunized with horse red blood cells (HRBC) were examined by determining the uptake of tritiated thymidine [( 3H]TdR) into the cells. Although the cells showed a basal proliferative response in the absence of antigen, addition of specific antigen increased the response sharply. Both the basal response and that stimulated by antigen disappeared if the cells had been previously treated with complement and anti-Iak antibody (AIak), anti-MAC-1 antibody (AMAC-1) or anti-Thy-1 antibody, but not anti-Ig antibody. Adding macrophages prepared from the peritoneal exudate cells of normal mice to AMAC-1-treated T-cell (i.e. Iak+ plus Iak- T-cell) cultures restored both of the responses, while adding them to AIak-treated T cells (i.e. Iak- T cells) only restored the antigen-specific response. These findings indicate that the basal proliferation is due to or dependent on the proliferation of Iak+ T cells, while the antigen-specific response is mainly due to Iak- T cells. Furthermore, interleukin (IL)-2 production was also examined. Immune T cells produced some IL-2 in the absence of antigen. The production by AMAC-1- or AIak-treated cells was impaired, but adding macrophages to the AMAC-1-treated cell cultures restored production. This result also suggests that the mode of IL-2 production by the Iak+ and Iak- cells is different. Proliferative responses of AMAC-1- or AIak-treated T cells to IL-2 were also examined. The AIak-treated cells dose-dependently responded to IL-2, while the response of Iak+ cells, which could be estimated by subtracting the response of AIak-treated cells from that of AMAC-1-treated cells, did not depend on the doses. These results indicate that in the immune peritoneal exudate the Iak+ T cells are functionally different from Iak- T cells.