Interleukin-12 (IL-12), originally called natural killer cell stimulatory factor or cytotoxic lymphocyte maturation factor, has potential for use as an immunomodulator in cancer therapy because it significantly retards the growth of some murine tumors. In this study, we analyzed the antitumor effects of lymphocytes stimulated in vitro with both recombinant IL-2 (rIL-2) and rIL-12. When IL-12 was added to mouse splenocytes (SPCs) or human peripheral blood monocytes (PBMCs) incubated with IL-2 for > 4 days, IL-2-induced cytotoxicity against glioma cells was augmented. In contrast, IL-12 inhibited IL-2-induced lymphokine-activated killer (LAK) cell activity when added concurrently to cultures. The concentration of IL-10 induced by IL-12 increased in the supernatant of human PBMCs costimulated with IL-2 and IL-12. Endogenous IL-10 augmented the cytotoxicity of SPCs stimulated with IL-2 or IL-12 or both. However, tumor-bearing mice treated with PBMCs stimulated with both IL-2 and IL-12 did not survive longer than those treated with PBMCs stimulated with IL-2 alone (LAK cells).