Histologic findings in mice with severe combined immunodeficiency (SCID) were remarkably uniform, consisting of lymphopenia, a rudimentary thymic medulla without cortex, relatively empty splenic follicles and lymph nodes, and undeveloped bronchial and gastrointestinal lymphocytic foci. Fluorescence-activated cell sorter studies revealed a few T cells (apparently nonfunctional) in thymus and spleen; interestingly, these cells seemed highly disposed to neoplasia, because thymic T-cell lymphomas were observed in 41 of 269 mice. No pre-B or B cells could be identified. Cells of the myeloid lineage appeared normal. Reconstitution of lymphoid tissues was achieved after intravenous injection of histocompatible bone marrow cells.