The influence of human recombinant interleukin-1 alpha (hrIL-1) on biosynthesis of collagenase and glycosaminoglycans was investigated with fibroblast-like cells of human chorionic membrane. hrIL-1 stimulated cells to produce procollagenase in a dose-dependent manner. Furthermore, it similarly accelerated both biosynthesis and secretion of hyaluronic acid in chorionic cells, but did not modulate the biosynthesis of sulfated glycosaminoglycans. Therefore, the relative concentration of hyaluronic acid vs total glycosaminoglycans increased significantly. These results are connected with the decrease in tensile strength observed in ruptured fetal membranes. Thus, it is proposed that IL-1 from effused leukocytes in fetal membranes plays an important role in connective tissue metabolism, especially in premature rupture of membranes with chorioamnionitis.