The effect of cobra venom factor on the developing and the established lesion of collagen-induced rat polyarthritis has been examined. In the developing lesion, decomplementation by cobra venom factor results in a delay in the onset of inflammation and a decrease in the radiological parameters of joint destruction. Under the conditions of the decomplementation, antibody titers to collagen are not decreased. In the established lesion, treatment with cobra venom factor has no effect, on either the inflammatory lesion or the various radiological parameters of joint destruction.