The single cells positive for placental glutathione S-transferase (GST-P), detectable in livers of rats soon after treatment with hepatocarcinogens, are possible 'initiated cells', the hypothesis tested in the present series of experiments. No low dose threshold was observed in male Sprague-Dawley rats at different single doses of diethylnitrosamine (DEN) although a plateau was reached between 160 and 200 mg/kg body weight. At the latter single dose approximately 12,400 positive cells/cm3 were observed immunohistochemically in rat livers after one week, the numbers then decreasing to week 8 and thereafter rising again. In the early stages single cells predominated but with time a gradual increase in mini-foci and larger lesions became evident. Application of selection pressure (feeding of diet containing 0.02% 2-AAF plus partial hepatectomy) to rats 2-24 weeks after single DEN-treatment resulted in the formation of large foci positive for GST-P, especially in the early stages, the growth response being less pronounced with time. The number of foci, on the other hand, was correlated with the number of single cells/mini-foci detected in hepatectomy tissue of the same individuals. These results suggest that the early GST-P positive populations could be the precursor for preneoplastic foci and nodules.