Carcinogenic and modification potential of 6-mercaptopurine (6-MP) was studied in a medium-term bioassay system for rat liver carcinogenesis. F344 male rats were initiated with a single dose (200 mg/kg body wt.) of diethylnitrosamine (DEN) i.p. and fed diets containing either 0.005% or 0.02% 6-MP with or without 0.05% phenobarbital (PB) for 6 weeks. Quantitative data revealed that 6-MP did not enhance the appearance of enzyme-altered preneoplastic foci and nodules even when administered at the highest dose (0.02%) despite showing an immunosuppressive effect and slight liver cell damage. Neither of the doses of 6-MP exerted any significant influence on the enhancing effect of PB when administered simultaneously in the medium-term-bioassay.