The tumorigenic potential of aluminum potassium sulfate [A1K (SO4)2 12H2O, APS], a compound which exists widely in the environment, was investigated in B6C3F1 mice. APS was administered in the diet for 20 months at dose levels of 1.0, 2.5, 5.0 and 10.0% (w/w). One group receiving the basal diet served as the control. Body weight gain in both sexes was decreased in the 10.0% APS treated group, and increased in the 1.0 and 2.5% APS treated groups. The survival rates at the end of the dosing period were 73.3% (male) and 78.3% (female) in the control group, and 86.7-95.0% (male) and 86.7-91.7% (female) in the APS treated groups. The survival rate showed a tendency to increase in both sexes in all the APS treated groups. In the tumor pathology, the incidence of hepatocellular carcinoma was significantly decreased in the males in the 10% APS treated group. The incidence of hepatocellular carcinoma was significantly decreased in females in all groups including the control group. As regards the nontumorous pathology, the incidence of myocardial eosinophilic cytoplasm showed a significant dose-dependent decrease in males in the APS treated groups. A comparison between the sexes revealed a significant decrease in the incidence of hepatocytic anisonucleosis, myocardial eosinophilic cytoplasm and acinar cell vacuolation of the submandibular gland in the females; and lymphocyte infiltration in renal cortex and pelvis, and vacuolation of cerebellar white matter were noted in the males. The results of the present study indicate that long-term administration of APS does not exert tumorigenic or any other toxic actions in B6C3F1 mice.