Abstract
A group of 150 female NMRI mice of the outbred stock Sut:NMRT was kept until they died naturally, at which time they were necropsied and examined histologically for spontaneous tumors. The natural life expectancy (median) was 782 days. Life expectancy was markedly reduced by mammary and pulmonary adenocarcinoma, and by tumors of the hypophysis. The spontaneous tumor rate was 58%. That is to say, 87 of the 150 mice had spontaneous tumors: 57 animals each had one tumor, 20 animals each had two tumors, and 10 animals each had three tumors. The organs most commonly affected by tumors were those of the lymphoreticular and haematopoietic systems, followed by the respiratory tract in second place and the breast in third place. Data reported in the literature generally show the same organ distribution, but the total tumor rate given is generally somewhat lower as the animals are seldom left alive until they die naturally (spontaneously).