We describe the identification of the GSTM1 null, GSTM1 A, GSTM1 B and GSTM1 A,B polymorphisms at the glutathione S-transferase GSTM1 locus using a single-step PCR method. Target DNA was amplified using primers to intron 6 and exon 7 with site-directed mutagenesis being used to introduce a restriction site in DNA amplified from GSTM1 *A, thereby allowing differentiation of this allele and GSTM1 *B. The accuracy of this approach in identifying the GSTM1 A, GSTM1 B, GSTM1 A,B and GSTM1 null polymorphisms was confirmed by comparison with, firstly, an established PCR method that distinguishes GSTM1 *0 homozygotes from individuals with the other GSTM1 genotypes and, secondly, GSTM1 phenotypes determined using chromatofocusing.