Supernatants harvested from peripheral blood mononuclear cells (PBMC) incubated either with the non-specific mitogen phytohaemagglutinin-P (PHA) or with the specific antigen tuberculin purified protein derivative for 72 h decreased collagen synthesis by dermal fibroblasts. PHA-induced mononuclear cell factors (PHA-MCF) responsible for decreases in collagen synthesis by dermal fibroblasts were localized by column chromatography on Sephacryl S-200 to fractions of 30,000-60,000 daltons. Proteolytic enzymes destroyed the activity of PHA-MCF, but after incubation with neuraminidase some activity of these factors remained. The activity of PHA-MCF was not inhibited by incubation with the monosaccharides L-fucose, L-rhamnose, N-acetylglucosamine, and alpha-methyl-D-mannoside but was partially destroyed by heating at 80 degrees C for 10 min. The factors were not mitogenic to PBMC. These factors did not appear to resemble any previously characterized factors produced by non-adherent mononuclear cells. The mechanism by which these factors decreased fibroblast collagen synthesis appeared complex. There was no detectable increase in the release of collagenase by fibroblasts, nor was a cytotoxic effect apparent. Increased PGE2 production by fibroblasts could not be related to the factor-induced decrease in fibroblast collagen synthesis.