The work presented here has concerned the study of early, as well as late, enzymic changes occurring during diethylnitrosamine-induced hepatocarcinogenesis and liver regeneration after partial hepatectomy in comparison with normal liver differentiation. Rank correlation analysis of the enzyme data suggested a step-wise retrodifferentiation i.e. that the liver during carcinogenesis first assumed a neonatal enzymic pattern before attaining a foetal enzymic state. Similar enzymic changes were observed in regenerating liver after partial hepatectomy; again there was a step-wise retrodifferentiation of enzymic pattern and at 3 days post hepatectomy the liver had an enzymic pattern similar to both foetal and neoplastic liver. However, in contrast to liver undergoing neoplastic change, the regenerating liver retained the capacity to undergo redifferentiation towards a normal adult biochemical pattern.