The aim of the present study was to verify whether the tumorigenic effect of a rat liver carcinogen, 2-acetylaminofluorene (AAF), and of a promoter of rat colon carcinogenesis, chenodeoxycholic acid (CDCA), could be detected with a single medium-term assay using as markers gamma-glutamyltranspeptidase (GGT)-positive foci in the liver and aberrant crypt foci (ACF) in colon mucosa. In rats given in the first 2 weeks of treatment both N-nitrosodiethylamine (NDEA), as initiator of liver carcinogenesis, and azoxymethane (AOM), as initiator of colon carcinogenesis, the subsequent 6-week feeding on a diet containing AAF (0.01%) produced a significant marked increase of the number and area of GGT-positive foci which is consistent with the results of long term assays. When rats initiated with both NDEA and AOM were fed for 6 weeks on a diet containing CDCA (0.1%) a significant increase of large ACF as well as of crypt multiplicity was observed, consistently with the promoting effect of CDCA in colon carcinogenesis. The results obtained in this preliminary study suggest that this medium-term assay might be able to screen both liver and colon carcinogens in rats.