Interindividual variation in the in vitro conjugation of methyl chloride with glutathione by erythrocyte glutathione transferase was investigated in 208 healthy males and females from the southern and central parts of Sweden. It was found that 11.1% of the individuals lacked this activity, whereas 46.2% had intermediate activity and 42.8% had high activity. This distribution of three phenotypes is compatible with the presence of one functional allele with a gene frequency of 0.659 and one defect allele with a gene frequency of 0.341. The proportion of non-conjugators in this Swedish material was considerably smaller than that previously found in Germany (Peter et al., Arch Toxicol 1989: 63, 351-355). The polymorphic distribution of another glutathione transferase, GST mu, was determined in the same individuals with a PCR method. No connection between the genotype for GST mu (GSTM1) and the glutatione conjugation with methyl chloride in erythrocytes was found.