Dermal fibroblast cultures from patients with progressive systemic sclerosis (PSS) synthesize up to 5 times more glycosaminoglycan (GAG) than normal cultures. In an in vitro model of fibroblast-lymphocyte interactions, we show that the supernatants of activated mononuclear cells (MNC) modulate GAG synthesis, as measured by the incorporation of 3H-glucosamine into GAG following incubation of the confluent fibroblast monolayers with active supernatant preparations. GAG accumulation was selectively increased up to 18 times in normal dermal fibroblast cultures. Cell viability was not affected, and 3H-thymidine uptake and cell numbers were depressed in cultures treated with the supernatants. In contrast to normal dermal fibroblast cultures, PSS fibroblasts responded to MNC supernatants by only a 1-2-fold increase in GAG. Supernatants of concanavalin A-activated PSS MNC had higher stimulatory activity than those of normal MNC. Supernatants made with MNC that had been depleted of monocytes on Sephadex G-10 columns were only minimally stimulatory. The GAG-stimulatory supernatants modulated the synthesis, but not the degradation of GAG. Gel filtration on a calibrated Sephadex G-100 column indicated the presence of stimulatory activity in both the 50,000 and 15,000 molecular weight fractions. These activities were trypsin-sensitive, but had different susceptibilities to heat. The active column fractions also contained interleukin-1 activity, as shown in an assay measuring proliferation of mouse thymocytes. Like our factors, interleukin-1 preparations increased GAG in normal and PSS dermal fibroblasts. Products of activated MNC may modulate normal and pathologic processes in human skin.