The presence and distribution of collagenase in experimental CCl4 cirrhosis of the liver in rats has been studied by immunohistochemical techniques. A monospecific anti-rat uterus collagenase antibody was raised in rabbits and used for indirect immunofluorescence staining of liver sections obtained from rats in both the reversible and irreversible stages of CCl4-induced cirrhosis. Collagenase is present assoicated with connective tissue septums as long as cirrhosis is reversible, and it is not detectable in the irreversible stage. In animals sacrificed during the transition between the reversible and irreversible stages of cirrhosis, collagenase appeared bound to the outer surfaces of connective tissue septums and was absent from the deeper portions. These observation suggest that the irreversibility of experimental CCl4 cirrhosis of the liver is associated with a disturbance in the mechanisms of collagen degradation, which may be a deficiency in collagenase activity, a change in the susceptibility of the substrate, or a combination of both factors.