Genomic DNA clones coding for polymorphic and monomorphic arylamine N-acetyltransferases (NAT) of human liver were isolated from a genomic DNA library, and their restriction maps and partial nucleotide sequences were determined. Messenger RNA for monomorphic NAT was coded in one exon, while mRNA for polymorphic NAT was coded in two exons; the 5'-noncoding region was located in one exon 8 kb upstream from another exon containing the coding and 3'-noncoding regions. Recently, we have shown that there are three types of polymorphic NAT gene; one of the genes corresponds to a high NAT activity, while the other two genes give rise to a low NAT activity. The restriction fragment length polymorphism (RFLP) was analyzed by Southern blot hybridization of genomic DNAs from homozygotes of the three polymorphic NAT genes using various fragments of the cloned NAT gene. RFLPs of polymorphic NAT gene were observed in coding and 3'-flanking region upon digestion with BamHI and KpnI.