Liver carcinogenesis with a single dose of aflatoxin B1 (7 mg/kg body weight) has been investigated in a group of female Wistar strain rats by repeated biopsies and necropsies. Another group received a subsequent intoxication with carbon tetrachloride by inhalation (approximately 200 doses) and another one was overloaded with riboflavin (25 parts/10(6) in drinking water). The frequency of hepatomata was almost equal in the aflatoxin and aflatoxin-carbon tetrachloride group. It was lowere in the riboflavin-aflatoxin group. In these 3 groups cirrhosis was never present in neoplastic livers. Megalocytosis was the first lesion observed. All tumoral livers had previous or concomitant megalocytosis. This modification was about as frequent, intense and widespread in aflatoxin-CCl4 and aflatoxin groups but appeared much earlier, as did the first hepatoma, in the aflatoxin-CCl4 group. It was less frequent, less intense and less widespread in the riboflavin-aflatoxin group than in the aflatoxin group. There was also a lower frequency of hepatomata in the riboflavin-aflatoxin group, but the difference was not significant due to the too small number of animals involved. The facts are not a proof of the existence of an obligatory link between megalocytosis and carcinogenesis since a slight megalocytosis was observed in the riboflavin group not affected by the neoplastic process. However, the simplest explanation of our results would be to consider that the potential tumour cells are located among the megalocytic cells, without admitting that every megalocyte is obligatorily a precancerous cell. CCl4 seems to act in shortening the time of appearance of megalocytosis. The protective effect of riboflavine should be regarded with more caution.