Inbred rats of the same age and sex were used for the immunochemical comparison of galactose cataract and normal lenses in order to determine if the cataractous process is associated with the formation of pathological lens protein. Identity of antigenic specificities of normal and galactose cataract lens crystallins as well as albuminoids could be demonstrated by crossabsorbed hetero and isoantisera. Since the inbred rats are a population of genetic replicas, the isoantisera can be considered as equivalents of autoantisera. The experiments showed that no pathological lens proteins are formed in cataract. This conclusion is significant because in the isoimmune sera differences between normal and pathological proteins cannot be masked by dominant interspecies differences as is the case with heteroimmune sera used so far. In immunoelectrophoretic titration, the cataract crystallins showed a somewhat lesser amount of pre-α, and a definite decrease of at least one specific β-crystallin antigen as compared to the normal crystallins. The potential role of these crystallins in cataracto-genesis is discussed. Physical differences between cataract and normal lens proteins were shown on disc gel electrophoresis. A fraction, which banded differently in cataract lens crystallins was shown to contain some of the heterogeneous α- and β-crystallins.