Conclusions In mice bearing spontaneous and transplanted mammary carcinoma it is possible by treatment with cortisone to induce a high frequency of massive necrosis with or without hemorrhage in the tumors. Cortisone administration in necrotizing doses is also followed by inflammatory lesions in lungs, liver and kidneys, which are usually fatal. Terramycin treatment before and during cortisone administration resulted in an abolition of the above-mentioned generalized reactions. It has not been possible with cortisone alone or with terramycin to obtain complete tumor destruction. It is pointed out that cortisone necrotizing treatment in conjunction with some other agent having differential damaging effects upon tumor .and host respectively, may yield better results.