We have previously reported a method for inducing natural suppressor (NS) cells by long-term culture of normal adult mouse spleen cells. The NS cells were further identified as mucosal or immature mast cells by morphology, cytochemistry, histochemistry and function. A cloned immature mast cell line was also confirmed to have NS activity. As NS cells, the cell line suppressed non-specifically the plaque-forming cell (PFC) response. The NS cell-free supernatant was partially enriched by chromatography and some fractions suppressed the PFC response and thymocyte proliferation. Heat treatment of the fractions failed to deplete the suppressive activity. The fractions were confirmed, by immunoblotting analysis, to contain transforming growth factor (TGF)-beta. Recombinant human TGF-beta was also able to suppress the PFC response and thymocyte proliferation. Neutralizing anti-TGF-beta reversed the suppression by both human TGF-beta and the fraction. From the above results, it is clear that mast cells displayed NS activity, at least partially, through the release of TGF-beta.