Fibroblasts derived from the involved skin of scleroderma patients frequently display a phenotype of supernormal collagen expression when cultured. Fibroblasts displaying this phenotype derived from seven patients were treated with relaxin (1-100 ng/ml) and interferon-gamma (1-100 U/ml), individually and in combination, to assess the relative abilities of these cytokines to down-modulate collagen synthesis and secretion. Scleroderma fibroblasts displayed varying sensitivities to both relaxin and interferon-gamma. Relaxin (100 ng/ml) decreased expression of collagen by six of seven lines tested from 8 to 59% compared to untreated cultures. Interferon-gamma (100 U/ml) depressed collagen secretion by all seven lines in a range from 7 to 89%. When relaxin and interferon-gamma were used in combination, relaxin augmented IFN-gamma-induced decreases in collagen secretion in four of seven lines. In three of these lines, the use of relaxin in conjunction with suboptimal doses of interferon-gamma resulted in decreases equivalent to or greater than that seen with a tenfold higher concentration of interferon-gamma. This study demonstrates the ability of relaxin to directly alter the excessive collagen-producing phenotype of scleroderma fibroblasts. In addition, in some cases, combining relaxin and interferon-gamma resulted in a cooperative effect in decreasing collagen expression by scleroderma cells in vitro.