The immuno-adjuvanticity of fresh plant juice of Viscum album and a polysaccharide fraction isolated from the berries was studied. Both delayed hypersensitivity and antibody formation against sheep red blood cells (SRBC) were enhanced after immunization of mice with a mixture of antigen and plant juice. Polysaccharide stimulated the immune response when administered 5 minutes before the antigen. Adjuvant-active doses of plant juice but not of polysaccharide caused non-specific inflammation as measured by paw swelling. Polysaccharide inhibited the pinocytic activity of the mononuclear phagocytic system initially but stimulated later on. Plant juice stimulated only for a short time. Both preparations caused splenomegaly while polysaccharide also increased the weight of the liver. The effects are compared with the activity of the Viscum preparation IscadorR.