We have previously shown that enhanced antitumor cytotoxicity is generated when thymocytes from melphalan (L-phenylalanine mustard; L-PAM)-treated MOPC-315 tumor bearers, but not thymocytes from normal mice, are added to the immunization culture of syngeneic normal spleen cells and MOPC-315 tumor cells (Bartik et al., Cancer Res., 47: 4848-4855, 1987). Here we show that normal spleen cells produce, upon stimulation with MOPC-315 tumor cells, helper-like factors which are sufficient for thymocytes from L-PAM-treated MOPC-315 tumor bearers, but not for thymocytes from normal mice, to develop antitumor cytotoxicity in response to stimulation with MOPC-315 tumor cells. Since one of the helper-like factors produced by in vitro-immunized spleen cells is interleukin 2 (IL-2), we assessed the exogenous IL-2 requirements for the development of anti-MOPC-315 cytotoxicity in thymocytes from L-PAM-treated MOPC-315 tumor bearers, relative to thymocytes from normal mice. Thymocytes from L-PAM-treated MOPC-315 tumor bearers were found to require a 10-fold lower concentration of recombinant IL-2 (rIL-2) than thymocytes from normal mice in order to develop antitumor cytotoxicity in response to stimulation with MOPC-315 tumor cells. The concentration of rIL-2 required for the development of anti-MOPC-315 cytotoxicity by thymocytes from L-PAM-treated MOPC-315 tumor bearers was also 10-fold lower than the concentration of rIL-2 required by thymocytes from untreated MOPC-315 tumor bearers or thymocytes from L-PAM-treated normal mice. In addition, at any concentration of rIL-2 employed, thymocytes from L-PAM-treated MOPC-315 tumor bearers developed a higher level of anti-MOPC-315 cytotoxicity than did thymocytes from normal mice, L-PAM-treated normal mice, or untreated MOPC-315 tumor bearers. The enhanced antitumor cytotoxicity exhibited by thymocytes from L-PAM-treated MOPC-315 tumor bearers, following in vitro stimulation with MOPC-315 tumor cells plus rIL-2, was evident not only against MOPC-315 tumor cells but also against other syngeneic plasmacytomas but not an allogeneic thymoma. In addition, thymocytes from L-PAM-treated MOPC-315 tumor bearers required less rIL-2 than thymocytes from normal mice to develop antitumor cytotoxicity in response to stimulation with MOPC-315-associated antigens but not in response to stimulation with an allogeneic antigenically unrelated thymoma (EL4).(ABSTRACT TRUNCATED AT 400 WORDS)