32P-Postlabelling was applied to study the distribution of adducts in white blood cells of foundry workers exposed to polycylic aromatic hydrocarbons. The distribution of the adducts among 63 workers followed an apparently trimodal pattern, which could relate to polymorphism in PAH metabolism. A modified postlabelling method is described and some parameters were tested for optimal labelling. The total volume of the polynucleotide kinase reaction is 2 microliters, which decreases exposure to radioactivity and costs of isotopes.