1. A genetic polymorphism of human liver arylamine N-acetyltransferase (NAT) enzyme activity leads to wide variation in the disposition of many drugs and potential carcinogens, resulting in differential susceptibility to chemical-induced toxicity. 2. During studies to determine the biochemical and molecular mechanisms underlying this pharmacogenetic defect, we cloned two human genes, NAT1 and NAT2, which encode the functional acetylating enzymes NAT1 and NAT2. 3. NAT1 and NAT2 are both expressed in human liver cytosol, the latter as two closely related isoforms NAT2A and NAT2B. 4. NAT2 gene locus is the site of the human acetylation polymorphism, because its products NAT2A and NAT2B selectively acetylate 'polymorphic' arylamine substrates (e.g. sulphamethazine), and since the liver content of these isozymes is markedly reduced in genetically slow acetylator subjects. 5. NAT1 shows marked kinetic selectivity for 'monomorphic' substrates (e.g. p-aminobenzoic acid) whose in vivo acetylation rates do not correlate with the acetylation polymorphism. 6. Despite the drastic reduction in NAT2A/B proteins in livers from phenotypically slow acetylators, levels of the NAT2 gene transcript are not altered. 7. Three common mutant alleles at the NAT2 gene locus have so far been identified, which may be detected by restriction fragment length polymorphism (RFLP) analysis on Southern blots or by allele-specific polymerase chain reaction amplification.