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The present report examines the behaviour of a slowly proliferating pure population of thymus-derived macrophages in long-term culture, regarding their ability to secrete interleukin-l (IL-I) and prostaglandin E2 (PGE2), to stimulate a mixed leucocyte reaction (MLR) and to lyse tumour cells in vitro. Following stimulation with LPS, high levels of IL-I were released to the medium. IL- I release was significantly augmented by the addition of indomethacin during stimulation. Thymus-derived macrophages constitutively secreted significant levels of PGE2. These cells served as excellent stimulators in a one-way MLR, substantiating the claim that pure populations of macrophages effectively stimulate an allogeneic response in vitro. Thym us-derived macrophages showed tumoricidal activity following activation with either high concentrations of LPS or suboptimal concentrations of LPS and 1-cell lymphokine. These findings portray a close interrelationship and reciprocal regulation between thymus-derived macro- phages and T lymphocytes. [ABSTRACT FROM AUTHOR]