The arginine-imbalanced diet used in this study was a diet of 27% casein supplemented with 8% arginine monochloride. There was no difference in the serum protein level between C3H/He mice fed a casein diet (N-mice) and mice fed on the arginine-supplemented diet (A-mice). The serum arginine level was increased in the A-mice compared with the N-mice. Significant inhibition of MM-48 tumor incidence was seen in the A-mice when the transplanted tumor was below 5 X 10(6) cells. Growth inhibitory effect against MM-48 tumor was significantly enhanced in the A-mice with i.p. injection of OK-432. Interferon production of spleen cells was increased in the A-mice compared with the N-mice. We speculate that the effect of arginine depends partially on the immune functions in the mice.