Repeated blood samples were obtained from volunteers who were occupationally exposed to polycyclic aromatic hydrocarbons in a Finnish iron foundry. Aromatic adducts were determined in white blood cell deoxyribonucleic acid of the subjects with the 32P-postlabeling technique after nuclease P1 treatment and butanol extraction, which showed no major difference. When repeated samples were analyzed, it appeared that the oldest ones displayed the lowest adduct levels, probably due to the instability of the adducts upon storage at -20 degrees C. The workers tended to maintain their adducts at a uniform level, a finding suggesting the contribution of host factors in the control of adduct levels.