A malignancy-associated human serum DNA-binding protein (C3DP protein), which was previously detected using dodecylsulfate gel electrophoresis, has been purified and characterized. This protein was isolated from human fetal cord serum by DNA-cellulose affinity chromatography, ammonium sulfate fractionations, DEAE-cellulose chromatography, and Sephadex gel filtration. The molecular weight of purified C3DP protein has been shown to be 135000 by ultracentrifugation, gel filtration, and dodecylsulfate gel electrophoresis. Dodecylsulfate gel electrophoresis following disulfide bond reduction has revealed that this protein is composed of three subunits having molecular weights of 74000, 40000 and 22000. Carbohydrate has been demonstrated to be attached to the 74000 and 22000 molecular weight components. Immunochemical studies have revealed that the C3DP protein is a fragment of human complement component C3, which closely resembles C3c.