Aldehyde dehydrogenase was measured in primary cultures of hepatocytes obtained with a two-step collagenase perfusion either from human hepatic tissue or from livers of Fisher rats. Basal enzyme activity declines gradually as a function of time in culture, but remains at all times higher when measured with propionaldehyde and NAD (P/NAD) than with benzaldehyde and NADP (B/NADP). Treatment of the cultures with 2 microM of 3-methylcholanthrene for four days significantly increased the B-NADP activity of human and rat hepatocytes (tenfold and eightfold respectively). In human hepatocytes 3-methylcholanthrene increases also the P/NAD activity, but to a lesser extent (twofold), compared to the B/NADP activity. Due to the significant enhancement of B/NADP activity in cultures of human and rat hepatocytes after application of 3-methylcholanthrene, the initial difference in the basal activity levels between the P/NAD and B/NADP forms diminishes or, in the case of human hepatocytes, is even inverted. These results show for the first time that aldehyde dehydrogenase activity is increased in cultured human hepatocytes. This biochemical property is preserved in human and rat hepatocyte cultures, despite the rather quick loss of the basal aldehyde dehydrogenase activity.