Rabbit antisera to bovine lens leucine aminopeptidase (LAP) have been prepared. These LAP-specific antisera cross-react with a component, LAP, in human lens homogenates. Bovine and human lens LAP are similar but not identical. Immuno-diffusion tests show that LAP is present in a vast majority if not all cataractous and normal human lens homogenates. Results from immunoelectrophoresis indicate that LAP is found in these homogenates as several metazymes as well as in an albuminoid - and possibly membrane-associated form. In contrast to many activity-based studies which imply that very little LAP is present in human lenses, micro-complement fixation tests indicate that the concentration of LAP in aged human lenses is similar to that found in calf lenses. Taken together, these data indicate that LAP undergoes age-related changes. These alterations of enzyme (or environment) result in an enzyme of markedly reduced activity, hence, the discrepancy between amount of LAP and the amount of enzymatic activity in aged human lenses.Active LAP is shown to enhance the rate of hydrolysis ofα-2 andα-B crystallins but notα-A2,βH,βLorγcrystallins. The ramifications of LAP inactivation with respect to cataractogenesis are discussed.