The mouse mutant nude is immunologically deficient due to congenital absence of the thymus. It will accept allografts and heterografts from closely and remotely related donors. Its humoral immune response to a number of other antigens is also imperfect. De novo malignancies can be induced in the nude by chemical carcinogen. The lack of a cell‐mediated immune response could be expected to be accompanied by a similar defect in the immunological surveillance mechanism against neoplasia. We have studied 11, 000 nude mice from birth to three months of age or to the normal life expectation of the animal. The total observation time within the period 1969–1972 was some 40, 000 months of mouse life. No malignancy was observed during this period. This striking observation is discussed in relation to de novo cancers following organ transplantation in man, and the high incidence of malignancies in patients with immune deficiency disorders. It is suggested that immunological surveillance—if existing—may represent a third expression of the immune system, separate from cell‐mediated and humoral immunity.