Fibroblasts from normal human adult skin were cultured in vitro in the presence and absence of different concentrations of pentoxifylline or a pentoxifylline analog, A81-3138 (10(-1)-10(3) micrograms/ml). Similar concentration dependent reductions in normal proliferation of fibroblasts in fetal calf serum-driven subconfluent cultures were detected following treatment with pentoxifylline or A81-3138. Fibroblasts assayed as confluent cultures produced sub-normal amounts of collagen, glycosaminoglycans (GAGs), and fibronectin in a fashion dependent upon the concentration of pentoxifylline. In contrast, fibroblasts exposed to pentoxifylline elaborated double the collagenase activity produced by normal, untreated fibroblasts. The reduced proliferation and reduced synthetic activities were not due to a lethal toxic effect on fibroblasts by pentoxifylline and A81-3138, nor was the reduction in collagen synthesis simply due to an inability to secrete newly synthesized intracellular collagen. Unlike pentoxifylline-induced inhibition of collagen and fibronectin production, which was detected only in cultures supplemented with serum, pentoxifylline inhibits, to a similar degree, both constitutive and serum-driven production of GAGs. The addition of IL1 beta (2.5 and 10.0 U/ml) to serum-driven fibroblast cultures resulted in greater proliferation, which was inhibitable by the presence of pentoxifylline and A81-3138 as anti-fibrotic agents in certain disorders of fibrosis.