The insoluble protein fraction of the ocular lens has been studied in four different species: the rat, dogfish, bovine, and human. These studies were performed in order to characterize the composition and formation of this protein in the ocular lens. On the basis of amino acid analyses, spectrophotometric determinations, thin-layer electrophoresis on Pevikon plates, and immunochemical data, it was concluded that in the rat and dogfish lens, the insoluble protein fraction derives mainly from the soluble gamma crystallin fraction. However, in the bovine and human lens, the insoluble protein fraction appears to derive mainly from the soluble alpha crystallin fraction.