This study presents data showing individual differences in aldehyde oxidase activity in human and monkey liver cytosols. When assayed with benzaldehyde as a substrate, a significant inter-subject variation in the activity was found in the human liver preparations. When assayed with N1-methylnicotinamide as a substrate, the inter-subject variation of the activity was also observed, but to a lesser extent compared with that of the activity with benzaldehyde. Similarly, variations in aldehyde oxidase activity were found in the monkey liver preparations when assayed with benzaldehyde or N1-methylnicotinamide. The present study suggested that at least two isozymes of aldehyde oxidase exist in the human liver preparations.