Six groups of female rats were fed diets containing 2% of one of six powdered seaweeds for 152 days and a basic diet for 59 or 60 successive days, and controls were fed the basic diet for the whole experimental period. The 7,12-dimethylbenz[a]anthracene was given to all rats intragastrically (20 mg/kg X 1), 27 days after the start of feeding. Diets with 3 weeds, Porphyra tenera (PT), Laminaria religiosa (LR) and L. japonica var. ochotensis (LO), showed an inhibitory effect on mammary tumorigenesis. Tumor incidences were 35% (7/20), 35% (7/20) and 50% (9/18), respectively, whereas that in the control group was 69% (20/29). There was a significant delay in the time to first palpable tumor in LR-fed and PT-fed rats (P less than 0.01). As for the tumor weight per rat in each group, it was significantly lower in the LR-fed group with a weight of 1.6 g, as compared with that of 16.3 g in the control group (P less than 0.02).