The effects of a prostaglandin synthesis inhibitor, indomethacin (Indo), on the growth of Lewis lung carcinoma (LLC) growing as primary subcutaneous tumors in either conventional C57BL/6 mice, T cell deficient nude mice, or natural killer (NK) cell deficient beige mice were studied. In conventional mice, Indo, when continuously administered in the drinking water, consistently and significantly inhibited, in a dose-related fashion, the growth of LLC implanted either subcutaneously in the footpad or in the inguinal region; however, the degree of inhibition of footpad tumor appeared to be greater than that of inguinal tumor. Maximum inhibition was found when Indo was initiated before detectable or measurable tumor developed. If Indo treatment was initiated after tumor growth was evident, then Indo was found to be less effective, although significant inhibition was still observed. Indo also effectively inhibited LLC growing either in the footpad or in the inguinal region of nude or beige mice. The degree of inhibition of both footpad and inguinal tumors in both these mice was comparable to that seen in conventional C57BL/ 6 mice, indicating that mature T cells, NK cells, or soluble products produced only by these cells are not involved in mediating or modulating the inhibitory effects of Indo on LLC growth. Although Indo treatment significantly inhibited LLC growth in vivo, continuous treatment of cultured LLC cells with Indo in vitro did not decrease the growth of cultured cells. These results indicate that the inhibitory effect of Indo in vivo is not the result of a direct inhibitory effect of Indo on these tumor cells. Lastly, this inhibitory effect of Indo in vivo could not be reversed or negated, not even in part, by the simultaneous, daily i.p. administration of 16,16-dimethyl-PGE2. This finding suggests that the inhibitory effect of Indo involves a mechanism other than the inhibition of prostaglandin E2production.