Homozygous nude (nu/nu) mice were inoculated ip with either highly malignant human bladder transitional cell carcinoma or human prostate adenocarcinoma. These animals were subsequently given injections of normal human T-lymphocytes to restore the known T-lymphocyte deficiency present in homozygous nude mice. Metastatic spread of the prostate and bladder carcinomas was evident in mice given human T-lymphocytes. Although tumor growth was observed at the sites of tumor inoculation, no tumor spread was observed in mice not receiving T-lymphocytes.