Pre- and postimmunization sera from six malignant melanoma and six ovarian carcinoma patients were used to investigate the humoral immune response to antigens expressed in extracts of allogeneic tumor cells and lysates of these same cells infected with virus. Nitrocellulose paper replicas of cell extracts, fractionated by polyacrylamide gel electrophoresis, were used as antigenic targets. Antibodies that bound to tumor cell antigens of defined molecular weight were identified with enzyme-linked probes specific for human immunoglobulins G, A, and M. Prior to therapy, all sera reacted with one or more antigens expressed by the unmodified tumor cells. Postimmunization sera from two malignant melanoma patients and one ovarian carcinoma patient reacted with antigens in extracts of uninfected tumor cells. These same antigens were not detected by preimmunization sera. Most postimmunization antibody responses were directed against antigens associated with the infecting virus itself and antigens found in extracts of virus-infected but not in extracts of uninfected tumor cells. These results suggest that treatment with lysates of virus-infected allogeneic human tumor cells elicits humoral immune responses against: (a) tumor cell-associated antigens; (b) antigens that are specifically virus associated; and (c) antigens that may be virus induced or virus modified cytoplasmic or nuclear antigens.