Acetylation by arylamine N-acetyltransferases (NATs) is a major route in the metabolism of numerous drugs and carcinogens. Recent studies suggest that the same enzymes also catalyze N,O-transacetylation and O-acetylation. A genetic polymorphism of clinical relevance divides the human population into slow and rapid acetylators of arylamines. Two human NATs, NAT1 and NAT2, have recently been characterized by protein purification, cloning, and functional expression of the respective genes; both were localized to chromosome 8. NAT1 codes for a protein with ubiquitous tissue distribution and a high affinity for p-aminobenzoic acid and p-aminosalicylic acid, so-called monomorphic substrates. NAT2 codes for a protein predominantly expressed in liver with a high affinity for sulfamethazine and other polymorphically metabolized drugs. NAT2 was analyzed at the level of protein, RNA and DNA derived from phenotyped slow and rapid acetylators. Two common (M1, M2) and one rare (M3) mutant allele were identified and their mutations characterized. A simple polymerase chain reaction-based DNA test can identify > 95% of mutant alleles and predict the phenotype.