A lipopolysaccharide (serratigen) and a polysaccharide (serratimannan) were isolated from Serratia marcescens, red strain No. 51. They were different from any other polysaccharides previously reported. The antitumor activity of these polysaccharides was determined. Serratimannan showed 63% tumor inhibition and serratigen 38%, at a dose of 150 mg/kg, against solid tumor of sarcoma-180 using ICR mice. The fraction obtained by removal of proteins from the crude extract by the Sevag method showed a high antitumor activity against solid tumor of sarcoma-180 using Swiss albino mice, but did not show so high an activity using ICR mice. In total packed cell volume method, these polysaccharides exhibited a high rate of antitumor activity against ascites tumor of sarcoma-180.