PURPOSE: To provide an anti-malignant tumor agent containing a lipopolysaccharide originated from bacteria belonging to O3 group of Klebsiella genus.

CONSTITUTION: The objective anti-malignant tumor agent is obtained by using a lipo-polysaccharide (KO3) originated from a bacterial strain belonging to O3 group of Klebsiella genus (e.g. Klebsiella pneumoniae LEN1 strain as an acapsulate mutant strain) as an active component. The agent is effective for the remedy of various malignant tumors e.g. adenocarcinoma such as cancers of digestive organs, squamous cell carcinoma such as carcinoma cutaneum, mammary carcinoma, acute lymphocytic leukemia, erythroneocytosis, Hodgkin's disease, lymphosarcoma, reticulum cell sarcoma, etc. It is administered orally in the form of granules, pills, powder, etc. at a dose of 100W2,000mg daily in 1W3 divided doses, or parenterally in the form of an injection continuously or intermittently at a rate of 0.3W8.0mg per dose. Although KO3 exhibits sufficient effect by itself, it has additive effect when used in combination with other anti- malignant tumor agent.