Peripheral blood mononuclear cells (PBMC) incubated either with phytohaemagglutinin-P (PHA) under serum-free conditions or with tuberculin purified protein derivative in the presence of 5% human blood group AB serum released into the culture medium factors that decreased collagen synthesis by dermal fibroblasts. These factors were first detected after incubation of PBMC with PHA for 24 h and were produced by non-adherent cells. The decrease in collagen synthesis was observed despite changes in conditions of fibroblast culture, the addition of a cross-linking inhibitor, and the use of different fibroblast cell lines.