To elucidate the molecular basis of human N-acetylation polymorphism, cDNA clones encoding human liver N-acetyltransferases (EC 220.127.116.11) were isolated from lambda gt10 cDNA libraries using the 32P-labeled cDNA of rabbit liver N-acetyltransferase recently cloned in this laboratory. Three types of cDNAs (D-14, O-7, and D-24) were isolated and their nucleotide sequences were determined, from which the amino acid sequences of human N-acetyltransferases were deduced. All the cDNAs coded for 290 amino acids. Between D-14 and O-7 cDNAs, there was only a single-base substitution in the coding region, which replaced glutamic acid in D-14 cDNA for glycine in O-7 cDNA. There were considerable differences between O-7/D-14 and D-24 cDNAs, with 80% homology in amino acid sequences. When the cDNAs were inserted into pEF321 expression vector and introduced into Chinese hamster ovary cells, N-acetyltransferase activity was expressed in three groups of the transfected cells. The activity in cells transfected with D-14 cDNA was only 9-17% of the activity in O-7 cells. Immunoblot analysis of the transfected cells indicated that the difference in the enzyme activity between O-7 and D-14 cells was possibly due to a difference in the amount of enzyme proteins. The substrate specificity of the expressed enzymes indicated that O-7 and D-14 cDNAs code for polymorphic N-acetyltransferase whereas D-24 cDNA codes for monomorphic enzyme. Southern blot analysis indicated that the polymorphic and monomorphic N-acetyltransferases were encoded in separate genes and that there was restriction fragment length polymorphism with KpnI digestion in the polymorphic N-acetyltransferase gene.