A human serum DNA-binding protein (C3DP) derived from complement component C3 has been found in elevated concentrations in the sera of individuals with malignant diseases. An assay system has been devised which reveals quantitative values of serum C3DP levels. Results obtained using this system indicate that normal human sera have an average C3DP level of 242 mug/ml (range, 40 to 146), whereas sera from individuals with active carcinomas have an average C3DP level of 242 mug/ml (range, 146 to 400). Sera from individuals with active leukemias, lymphomas, and melanomas all had elevated levels of C3DP, whereas sera from individuals with polycythemia vera or other nonmalignant diseases had normal or only slightly elevated C3DP concentrations. No tissue specificity seems to be required for malignant growths to result in elevated C3DP serum concentrations. The levels of C3DP in 79% of the individuals who experienced disease remission were found to decline to normal values, concurrent with the disease regression. Patients who did not respond to therapy regimens retained high C3DP levels.