An insoluble fraction isolated from the bacterium Corynebacterium granulosum with potential cancer immunotherapeutic activity. As a non-specific immunostimulant, Corynebacterium granulosum P40 activates the reticulo-endothelial system; induces the production of certain cytokines; enhances macrophage activity; and potentiates a delayed-type hypersensitivity response when co-administered with an antigen.