This invention relates to an antitumor agent comprising as a main ingredient a chemically purified cell walls of microorganisms belonging to genus Bificobacterium having physical integrity of cell wall structure thereof, obtained without being subjected to chemical and/or physical destruction of said cell wall during removal of intracellular substances of said cell, and a process for manufacturing the same, which comprises the steps of; (a) treating said cells with surfactants, (b) treating said cells with proteases, nucleases, organic solvents and dilute acid, to remove intracellular substances out of said cells without destruction thereof, and (c) separating a chemically purified cell walls having physical integrity of cell wall structure thereof.