Congenitally athymic nude mice (nu/nu) are presented as a model for the study of cell-mediated immunologic deficiencies. These mice possess a vestigial thymus which is incapable of producing mature T-cells as shown by a decreased lymphocyte population carrying the theta antigen and depleted 'thymus-dependent' areas in their peripheral lymphoid organs. However, they have T-cell precursors in their bone marrow. Nude mice lack 'thymosin', a thymic factor secreted by the epithelial cells of the thymus. They also have impaired T-cell functions as demonstrated by an absence of delayed hypersensitivity reactions, an inability to reject skin allografts and a decreased antibody response to some T-dependent antigens. But their response to T-independent antigens is normal. The presence of circulating auto-antibodies and immune-complex type glomerulonephritis in nude mice reinforces the hypothesis that spontaneous autoimmunity and B-cell hyperactivity are associated with T-cell deficiency. On the other hand, they are not especially susceptible to spontaneous tumor development. Correlations are made between the pathologic findings in nude mice and some congenital cellular immunologic disorders in man.