Pretreatment of mouse splenocytes with Shigella lipopolysaccharide and concanavalin A followed by 50 ng/ml of cimicifugoside resulted in a 69% and 31% inhibition of blastogenesis compared to controls. The plaque forming colony assay using sheep erythrocytes (SRBC) showed a decreased number of plaque forming colonies after exposure of the splenic cells to 1 μg/ml of cimicifugoside. Cimicifugoside, 0.1 mg/mouse i. p. suppressed the anti-SRBC response in the plaque forming assay. The major inhibition of the antibody response occurred when cimicifugoside was administered 1 day before the primary immunization with SRBC. The delayed type hypersensitivity to picryl chloride was suppressed after i. v. administration of cimicifugoside, 1.0-2.0 mg/mouse. The immunosuppressive activity of cimicifugoside is preferentially directed toward B-cell function with larger doses being required for suppression of T-cell function.