NMRI mice normally died 3 weeks after an intraperitoneal transplantation of 2 × 104 Ehrlich ascites tumour cells. If they were, however, injected intraperitoneally with about 380 pneumococci 18 h after tumour transplantation and if theinfection was cured with penicillin, almost 60’% of the animals failed to develop tumour. This result is highly significant (p = 0.00079).