A protein-bound polysaccharide from mycelia of Coriolus versicolor PS-K, clinically used as an immunomodulator, has been shown to restore the decreased cellular immune response and to exhibit host-mediated antitumor activity. In this experiment, PS-K was found to increase serum complement level in guinea pig and in human without malignancy, when hemolytic assay of complement was performed using sensitized sheep erythrocytes for the classical pathway activity and unsensitized rabbit erythrocytes for the alternative pathway activity. Assay of complement components revealed increase in C3 level in guinea pig, but no significant changes in C1q, C4, C3, properdin, C3 activator, and C1-inhibitor in human, while C5 and C9 were depressed. Conversion of beta 1C to beta 1A was observed in the 7th day's plasma of these patients by crossed immunoelectrophoresis. Biosynthesis of guinea pig C3 was accelerated by administration of PS-K, but that of C4 was not affected. These evidences suggested that PS-K might potentiate immune response of the host by elevating serum complement level, in addition to the activation of the complement system.