Three human phenol sulfotransferases, provisionally named SULT1A1, 1A2 and 1A3, show 91–96% homology of their amino acid sequences and are encoded by neighbouring gene loci. Functional genetic polymorphisms are known for two of these sulfotransferases. InSULT1A1, a G to A transition leads to an Arg213to His exchange and eliminates aBsp143II restriction site. SULT1A1*His shows lower enzyme activity and thermostability than SULT1A1*Arg. InSULT1A2, an A to C transversion causes an Asn235to Thr exchange and introduces aBpiI restriction site. Enzyme SULT1A2*Thr is less active than SULT1A2*Asn. These substitutions were detected by restriction fragment length polymorphism analyses of genomic sequences amplified by polymerase chain reaction. Despite the high similarity between the different humanSULT1Agenes, it was possible to amplify specifically the polymorphic parts of eitherSULT1A1or1A2, but not the homologous sequences of the otherSULT, by setting the forward primer into intron 6. DNA from 300 adult male Caucasian subjects was analysed. Allele frequencies were 0.63 and 0.37 forSULT1A1*Argand*His, and 0.62 and 0.38 forSULT1A2*Asnand*Thr, respectively. The frequency of the haplotypeSULT1A1*Arg/SULT1A2*Asn(0.61) was nearly as high as the allele frequencies of its components. The same was observed for the haplotypeSULT1A1*His/SULT1A2*Thr, whose frequency was 0.35. In contrast, haplotypes1A1*Arg/1A2*Thrand1A1*His/1A2*Asnwere very rare. Their frequencies (0.02 each) were less than 10% of the figures expected in an independent distribution. The results demonstrate a strong association of the alleles producing the more active enzyme variants (SULT1A1*Arg and SULT1A2*Asn) and of those encoding the less active variants (SULT1A1*His and SULT1A2*Thr).