The influence of polymorphic glutathione S-transferases mu (GSTM1) and theta (GSTT1) on the rate of chromosome aberrations (CA) in peripheral lymphocytes of 30 pesticide-exposed floriculturists and 32 control subjects was studied. Pesticide exposure was not associated with elevated frequencies of CA. Among cigarette smokers, a statistically significant (p = 0.026) increase in baseline CA frequencies was observed in subjects with a homozygous deletion of the GSTM1 gene (GSTM1 null, n = 36) in comparison with those having at least one copy of the gene (GSTM1 positive, n = 26). This effect was mainly due to an excess of chromatid-type aberrations (p = 0.006). In addition, the few individuals (n = 5) deficient for both GSTM1 and GSTT1 showed significantly higher (p = 0.012) CA counts than GSTM1 positive GSTT1 nulls. Despite the limited number of subjects genotyped, the results seem to indicate an association between smoking induced CA frequencies and GSTM1 polymorphism, and a possible interaction between the GSTM1 and GSTT1 genotypes. The findings may be explained by the reduced detoxification capacity of GSTM1 null and GSTT1 null individuals.