Transforming growth factor (TGF)-beta is a multifunctional cytokine, which in mammals exists in three isoforms (TGF-beta1, 2 and 3). It is synthesized by a variety of cells including macrophages, and exerts potent immunoregulatory effects such as the inhibition of Th1 development and the suppression or reversal of IFN-gamma-induced macrophage activation. In this study we analyzed the effect of IFN-gamma on the production of TGF-beta1 by thioglycolate-elicited mouse peritoneal macrophages under serum-free conditions. Untreated macrophages released TGF-beta1 in its latent form, which became detectable in a capture ELISA specific for active TGF-beta1 after acid activation of the culture supernatants. Treatment with IFN-gamma reduced the amount of latent TGF-beta1 in the culture supernatants in a dose-dependent fashion. The effect of IFN-gamma was confirmed by a newly developed Western blot system for the detection of mouse TGF-beta1 protein. IFN-gamma only weakly (16-24 %) reduced the levels TGF-beta1 mRNA at early and late time points of stimulation, and no evidence was obtained that IFN-gamma suppresses the secretion of latent TGF-beta1. Thus, inhibition of TGF-beta1 production by IFN-gamma is most likely due to decreased synthesis and/or stability of the TGF-beta1 protein, and might be important for the generation of fully activated macrophages and a Th1 response.