Corynebacterium parvum strain 10390 is an antitumor agent and stimulant of the reticuloendothelial system and produces a soluble antigen towards the end of its growth cycle. This material, which is a cell wall component and can also be released from the organism by acid or alkaline hydrolysis, has been purified. It is an acidic polysaccharide of molecular weight 100,000 to 150,000 and contains galactose, glucose, fucose, N -acetylgalactosamine, N -acetylglucosamine, uronic acids, sialic acids, and a small proportion of amino acids. The antigen gives a precipitin reaction with antisera raised against the whole organism and also binds to animal cells. The antigenic determinants are extremely resistant to oxidation, reduction, and enzymatic and chemical hydrolysis, but the single cell-binding site is destroyed by alkali and also by Helix pomatia digestive juice, alginase, and neuraminidase without substantially affecting the molecular weight. This site is inaccessible until the molecule is released from the cell surface. The possibility that the soluble antigen is the biologically active fraction of C. parvum is discussed.