Skin fibroblasts were cultured from 15 patients with primary immunodeficiency diseases associated with a high cancer risk, including sex-linked agammaglobulinemia, IgA deficiency, variable immunodeficiency, ataxia-telangiectasia (cerebellar malfunction and abnormalities of blood vessels and immune response), Wiskott-Aldrich syndrome (low platelet count, eczema, and abnormal immune mechanism), and severe combined system (cellular and humoral) immunodeficiency. Fourteen of 15 cell strains were found to have low or regular susceptibility to transformation with the tumor virus, simian virus 40. The data are consistent with the view that the frequent occurrence of malignancy in patients with primary immunodeficiency is due to abnormalities of the immunologic surveillance mechanism.