A hypoalbuminemic substance was purified from mouse Ehrlich ascites carcinoma cells by ammonium sulfate fractionation, adsorption on DEAE-cellulose, gel filtration on Sephadex G-200, treatment with butanol, ultracentrifugation, and gel filtration on Sephadex G-75. Electrophoresis of the substance on polyacrylamide gel containing sodium dodecylsulfate gave a single band which corresponded to a molecular weight of 53000. It showed hypoalbuminemic, hypoproteinemic, reticuloendothelial system-stimulating, and leukocyte-lowering effects when given to mice by intraperitoneal injection. Of these effects, only the leukocyte-lowering effect disappeared when it was administered to adrenalectomized mice. The vascular permeability-increasing activity concentrated into the fraction unadsorbed on DEAE-cellulose and distinctly separated from the hypoalbuminemic activity. The purified substance was composed of 28.9% of DNA, 24.9% of hexose, 24.6% of RNA, 14.6% of amino acid residues, and 1% of hexosamine. The hypoalbuminemic activity was decreased markedly by the partial digestion with Pronase, trypsin, or DNase, whereas the treatment with RNase or the butanol extraction exerted no influence on the activity.