Antitumor activity of the whole cell and slime of an aquatic sheathed bacterium, Sphaerotilus natans IAM 12068, against ascites form of Ehrlich carcinoma in ddY mice was investigated. Intraperitoneal injection of whole cells and the slime fraction showed remarkable antitumor activity against mice inoculated with 10(4) to 10(5) tumor cells, the slime fraction being more effective. To examine the chemical nature of the active principle in the slime fraction, separation by Sepharose 4B gel filtration was carried out and two fractions designated as GF-P-1 and GF-P-2, which are mainly composed of protein, carbohydrate, and lipid, were obtained. GF-P-1 fraction, which contains large amounts of fucose and unidentified sugar as neutral sugar, showed marked antitumor activity at half the dose of the slime fraction, whereas the antitumor activity of GF-P-2, which is composed mainly of protein, was weak. This finding indicates that GF-P-1 fraction of S. natans slime may be a main active principle. The consistently demonstrable antitumor activity of GF-P-1 was abrogated by treatment of mice with silica, an anti-macrophage agent, suggesting that the antitumor activity of GF-P-1 depends on the activation of macrophages.