Proliferation of fibroblasts is a serious problem in ocular trauma and surgical wound healing. Depending on the location of the injury, the growth of fibroblasts can lead to different problems. In glaucoma filtering surgery, fibroblast proliferation may contribute to scar tissue formation and premature wound closure. Fibroblastic growth in proliferative vitreoretinopathy may lead to the formation of preretinal membranes, which can contract, causing retinal detachment. In an effort to find a more effective method of inhibiting ocular fibroblast proliferation, we have investigated the effect of heparin, a sulfated polysaccharide, on the proliferation of fibroblasts obtained from the sclera of donor eyes. Heparin inhibits the incorporation of 3H-thymidine in a dose-dependent manner in the presence of fetal bovine serum (FBS). This inhibition is partially reversed by endothelial cell growth factor (ECGF). The heparin antagonist protamine sulfate causes a reversal of heparin inhibition and, in some instances, a significant increase in 3H-thymidine incorporation compared to serum controls. Heparin was equally effective in inhibiting cell proliferation in control and heparin-protamine sulfate-pretreated medium. These results were apparently unrelated to a direct toxic effect on cells, as a Trypan Blue exclusion assay showed no significant difference in viability when heparin treated cells were compared to control cells. Direct cell counts showed that heparin was effective in inhibiting cell proliferation over a long time period, but only if it was reinstilled every 2 days. Heparin treatment shows promise as a method for controlling fibroblast proliferation in the eye.