The inhibitory effect of butylated hydroxyanosole (BHA), disulfiram, calcium chloride, and polyvinylpyrrolidone (PVP) on the intestinal and urothelial carcinogenicity of bracken fern (BF) was determined in albino rats. Of 10 groups of rats, one group received a normal diet, one received a BF-containing diet (one-third of diet by weight), four received a normal diet with one of the following supplements, and four received BF-containing diet with one of the following: BHA (5 mg/g diet); disulfiram (5 mg/g diet); PVP (50 mg/g diet); or calcium chloride (20 mg/g diet). At 12 months of the experiment, the following results were noted: in the BF-treated group, 30 rats (100%) exhibited intestinal tumors and 22 of 30 (73%) urinary bladder tumors. In the BF-BHA group, 15 of 20 rats (75%) showed intestinal tumors and 12 of 20 rats (60%) urinary bladder tumors. Of the 16 rats in the BF-disulfiram group, 12 (80%) had intestinal and 10 (62.5%) had urinary bladder tumors. In the BF-calcium chloride group, intestinal tumors arose in 16 of 23 rats (70%) and urinary bladder tumors in 4 of 23 rats (17%), while in the 28 BF-PVP rats, 26 (93%) exhibited tumors of the intestine and 5 (18%) tumors of the urinary bladder. Dietary BHA, disulfiram and calcium chloride decreased the incidence of intestinal tumors by about 25--30% (p less than 0.01). Similarly, PVP and calcium chloride inhibited BF-induced urinary bladder carcinogenesis by about 80% (p less than 0.001). No tumors were detected in groups receiving either normal diet or normal diet supplemented with BHA, disulfiram, calcium chloride or PVP.