Failure of a glaucoma filtering operation mainly results from scarring at the filtering wound, and postoperative proliferation and migration of fibroblasts play an important role histologically in the formation of scar tissue. As an inhibitory agent for fibroblast proliferation, gamma-interferon has been introduced, and the application of gamma-interferon following filtering surgery is now being made on a trial basis. We studied the effect of gamma-interferon histologically on the fibroblast proliferation and collagen synthesis occurring at the filtering site by comparing the effect of gamma-interferon on the experimental group with that of 5-fluorouracil on the control group, using 10 rabbits (20 eyes) after posterior lip sclerectomy. Both groups showed similar flat and diffused bleb grossly and also showed a similar inhibitory effect on fibroblast proliferation and collagen fiber synthesis histologically. Our findings seem to justify the clinical use of gamma-interferon. Further studies on adequate dosage, method of administration, and local and systemic complications would be desired.