We have already reported that TGF-beta could be involved in the inhibitory effects of negatively charged liposomes composed of phosphatidylserine (PS-liposome) on the production of nitric oxide (NO) by mouse peritoneal macrophages stimulated with LPS [Biochem. Biophys. Res. Commun. 281 (2001) 614]. In this paper, we explored the mechanism by which PS-liposomes promote the production of TGF-beta and the involvement of MAP kinases. When macrophages were treated with PS-liposomes, extracellular signal-regulated kinase (ERK), a member of MAP kinase superfamily, was activated quickly and potently. However, no activation was observed with p38 MAP kinase. TGF-beta production was completely inhibited by U0126, a specific inhibitor for ERK. Furthermore, TGF-beta neutralizing antibody and U0126 decreased the inhibitory effect of PS-liposomes on NO production by macrophages. These findings suggested that TGF-beta is the factor produced by PS-liposomes that suppresses production of NO, and the ERK signaling pathway is intimately involved in TGF-beta production by macrophages following treatment with PS-liposomes.