The immune response to a protein antigen, keyhole limpet haemocyanin, was studied in fourteen normal subjects and twenty-one patients with solid tumours. Immunological responsiveness was assessed by intracutaneous skin testing, by haemagglutinin titres and by in vitro blast transformation. No significant difference was found in the kinetics or magnitude of the immune response among subjects immunized with 0· 01, 0· 10, or 5· 0 mg. Delayed hypersensitivity to keyhole limpet haemocyanin developed in thirty-two of thirty-four skin tested; a positive antibody titre occurred in all; and thirty-one of thirty-five had positive in vitro responses. Patients in good general condition (Group 1) had significantly greater delayed hypersensitivity and antibody responses than the normals but similar in vitro responses. All immunological parameters were depressed in the patients with advanced neoplastic disease (Group 2).