[Induced resistance of experimental tumors to nitrosomethylurea].

A nitrosomethylurea (NMU)--resistant strain of transplantable solid mammary gland tumor, which arose spontaneously in a F1CBA X S57Bl mouse, was developed, following successive transplantations of tumors from animals which received single intraperitoneal injections of 60-80 mg/kg of NMU. The basic tumor showed a high sensitivity to NMU (4th generation). Resistance to NMU was obtained after the 8th transplantation of tumor from NMU-treated animals: When the treatment of animals with NMU-resistant tumors was continued, tumor growth was stimulated and the time of survival became shorter. Tumors with induced resistance to NMU retained sensitivity to cyclophosphamide and sarcolysin and were refractory to dimethylnitrosourea treatment.