Delta aminolevulinic acid dehydratase (ALAD) plays an important role in lead poisoning. This study was carried out to examine the effects of ALAD gene polymorphism (G177C) on %Pb-P(plasma lead)/Pb-B(whole blood) ratio in 142 subjects environmentally exposed to lead. Genotypes for the ALAD G177C polymorphism were determined by PCR and restriction fragment length digestion. Pb-P and Pb-B were determined by inductively coupled plasma mass spectrometry and by graphite furnace atomic absorption spectrometry, respectively. The allele frequencies for ALAD1 and ALAD2 alleles were 0.897 and 0.103, respectively. We combined both ALAD 1-2 and ALAD 2-2 genotypes together (ALAD 1-2/2-2 group) and compared with the ALAD 1-1 genotype group. While no significant differences were found in Pb-B, subjects from the ALAD 1-2/2-2 genotype group presented significantly higher Pb-P concentrations and %Pb-P/Pb-B ratios (0.89+/-0.07 microg/l, and 1.45+/-0.10%, respectively) when compared with subjects from the ALAD 1-1 genotype group (0.44+/-0.05 microg/l, and 0.48+/-0.02, respectively; both P<0.0001). The higher %Pb-P/Pb-B ratios in carriers of the ALAD-2 allele compared with noncarriers indicate that ALAD 1-2/2-2 subjects are probably at increased health risks associated with lead exposure.