To determine the predictive value of the IgG anti-cardiolipin antibody test for malignant tumor, two groups of the patients of Kurume and Kyushu University Hospitals were examined by quantitative enzyme linked immunoadsorbent assay (ELISA) for their serum anti-cardiolipin antibodies. With the first group of the patients consisting of a mixture of benign diseases and malignant tumors, the antibody was positive at 57% (39/72). With the second, malignant tumor group, 38% (19/50) was positive for the antibody. When examined for the specificity of the antibodies by a binding inhibition assay, it was shown that the antibodies in the tumor patients were less specific to cardiolipin: antibodies to cardiolipin in tumor patients were blocked by the specific antigen, cardiolipin, in a lesser degree than those in the control syphilis patients, and highly cross-reacted with 2 related phospholipids, i.e., phosphatidyl glycerol and phosphatidic acid. These findings obtained in this study suggest that ELISA for cardiolipin has the value for the tumor diagnosis, supplementing already established assays.