The study concerns comparisons betweennudemice of three genetic backgrounds (BALB/c, C3H&C57/BL) after the third cycle of a gene transfer, and normal members of the respective three inbred strains. The following parameters were considered: body and organ weight, haematological values, differential counts of bone marrow and peripheral blood cells, serum proteins (total and fractionated), and detailed histological appearances of tissues and organs. No thymus tissue was found in anynudemouse. Lymphoid tissue in the spleen, Peyer's patches and lymph nodes was of characteristic histological appearance with lymphocytic depletion. Additionally, marked difference in phagocytic activity in Peyer's patches was observed. Such activity was meagre or absent innudemice. The possible relationship of this finding with low IgA levels is discussed. Peripheral blood lymphocyte counts innudes, irrespective of genetic background, were uniform and consistently very low. It is proposed that these lymphocytes represent a stable B‐cell population. Lymphocyte counts in the bone marrow were somewhat low in allnudes.No quantitative differences in serum protein values could be demonstrated by the methods used. Histological study demonstrated inflammatory changes in one or more organs in 11 of the 24 nude mice, despite the fact that all animals were apparently healthy. No changes were found in the normal mice. The mice were reared under spf (specified pathogen free) conditions. The insufficiency of current spf‐definitions in relation tonudemice is stressed. The infections were always ascending, and frequently involved the genital tract. The significance of possibly impaired IgA secretion is discussed. The genital infections are considered in relation to the infertility ofnudefema