The NAT2 gene encodes for a polymorphic arylalkylamine N-acetyltransferase and thus accounts for the human N-acetylation polymorphism. By a NAT2-specific primer set we have screened a human chromosome 8-specific cosmid library. A positive cosmid clone was mapped by fluorescence in situ hybridization to 8p22. The polymerase chain reaction followed by restriction analysis of the PCR product was used to identify allele 2 to be contained in the cosmid clone.