Immunological characterization of nude rats (rnu/rnu) of outbred hooded stock was attempted. Antibody responses of IgM and IgG classes against thymus-dependent antigen, sheep red blood cells (SRBC), were detected in heterozygous (+/rnu) rats but were totally lacking in homozygous (rnu/rnu) nude rats. When mouse Sarcoma 180 was transplanted, tumor cells were not taken by heterozygous rats but grew in nuce rats for more than 30 days showing marked tumor growth. Mitotic response to T-cell mitogens, concanavalin A and phytohemagglutinin, was seen in spleen cells of heterozygous but not of nude rats. It was concluded that the T-cell function is lacking in these nude rats.