Interleukin-1 (IL-I) is known to inhibit proliferation in some tumor cells. This proinflammatory cytokine also induces nitric oxide production in a variety of cell types. In the present studies we determined if nitric oxide is involved in IL-1 induced growth inhibition in spontaneously transformed hamster embryonic fibroblasts (STHE cells). Both IL-la and IL-lp were found to stimulate nitric oxide production and to reduce 'H-thymidine (TdR) incorporation in high density cultures of STHE cells. However, maximal cytostasis was observed at least 24 h before significant amounts of nitric oxide accumulated in the cultures. In addition, doses of IL-I which were too low to stimulate nitric oxide synthesis were effective in inducing cytostasis. Furthermore, in low density cultures of STHE cells, IL-1 inhibited DNA synthesis without inducing nitric oxide production. The nitric oxide synthase inhibitor N"-rnonomethyl-I-arginine (L-NMMA) had no effect on proliferation of cells plated at low density. In contrast, L-NMMA treatment resulted in a 40-60% reduction in IL-1 induced cytostasis in high density cultures. Neutralizing antibodies to IL-1 were found to completely block IL-1 induced cytostasis and nitric oxide production in cells plated at both densities. Although anti-IL-1 a and anti-IL-1 p antibodies were highly specific and did not cross react, anti-tumor necrosis factor-a (TNF-a) antibody was able to partially suppress activation of STHE cells by both IL-la and IL-I 0. These data suggest a potential involvement of endogenous TNF-a in IL-1 induced cytostasis and nitric oxide production. Exponentially growing STHE cells produced six-times less nitric oxide than nonproliferating cells. A ten-fold excess of I-arginine was found to stimulate nitric oxide synthesis, an action that was independent of the rate of cellular proliferation. Taken together these data suggest that nitric oxide is not a major mediator of IL-I induced cytostasis in STHE cells. Moreover, cytostasis appears to be required for nitric oxide synthesis in these cells. o 1996 ~i l e y -~i s s , Inc.