Human glutathione S-transferase (GST) M1 and T1 enzymes exhibit genetic polymorphism, with a percentage of normal individuals exhibiting a homozygous deletion of the relevant genes. We established a differential polymerase chain reaction (PCR) technique to simultaneously characterize inactivating mutations responsible for the null alleles ofGSTM1andGSTT1.Primers forGSTM1, GSTT1,and for β-globin (as a positive control) were used to simultaneously amplify all three gene products from leukocyte DNA from 416 normal healthy human volunteers. IdenticalGSTM1andGSTT1genotypes were obtained using nine samples processed either separately or simultaneously forGSTM1andGSTT1.The frequency of the null genotype forGSTM1was higher in whites (114/213 or 53.5% vs 56/203 or 27.6%, p<0.001) and forGSTT1was higher in blacks (49/203 or 24.1% vs 32/213 or 15.0%, p=0.019). The observed frequency of the ‘double null’ genotype for bothGSTM1andGSTT1was not significantly different from that predicted if both polymorphisms were independent (p = 0.102) and did not differ by race (p = 0.120) or sex (p = 0.800). There was a higher frequency of theGSTM1null genotype among females than males (92/202 or 45.5% vs 78/214 or 36.4%, p = 0.049). These results demonstrate that this PCR method is a simple and reliable tool to simultaneously characterize bothGSTM1andGSTT1null genotypes.