Environmental related diseases due to occupational carcinogens and toxic substances are a serious problem particularly in developing countries. The glutathione S-transferase system is fundamental for the detoxification of numerous carcinogens and mutagens. The individual inherited susceptibility to chemical carcinogenesis due to glutathione S-transferase mu (GSTMl) and theta (GSTTl) varies significantly among distinct ethnic groups. In this study we determined the prevalence of the null genotype of the GSTMl and GSTTl genes among individuals from three distinct Brazilian racial groups using a multiplex- PCR methodology. The results showed that the highest prevalence of the null genotype for the GSTMl occurred among Caucasians (55%, allele frequency = 0.74), followed by 33% among Brazilian Black subjects (allele frequency = 0.57). and 20% among Amazonian Indians (allele frequency = 0.45). For GSTTl a homogenous distribution of the null genotype was found among Caucasian and African descendants (18.5 and 19% homozygotes, respectively, allele. frequency = 0.43). with a lower prevalence among Amazonian Indians (1 1% of homozygotes, a!lele frequency = 0.34). Whether the deficiency of the GST system contributes to a predisposition to environmental related carcinogenesis in I specific popuiations in Brazil remains to be determined.