The cancer initiating potential of fusarin C, a mutagen produced by Fusarium moniliforme strain MRC 826 was investigated on mouse skin using 12-O-tetradecanoylphorbol-13-acetate as promoter. In neither of these models did fusarin C act as a cancer initiator. Culture material of strain MRC 826, which previously was found to be hepatocarcinogenic in rats, exhibited cancer promoting activity in rat liver using diethylnitrosamine (DEN) as initiator and the induction of gamma glutamyltranspeptidase (GGT)-positive foci as end-point. The culture material of this fungus could also induce the formation of GGT positive foci without DEN initiation. These results seem to indicate that fusarin C is not involved in the hepatocarcinogenic activity of F. moniliforme strain MRC 826.