The adjuvanticity of the mistletoe preparation Iscador was investigated. The cellular response to sheep red blood cells (SRBC) was augmented after intracutaneous immunization with antigen and different doses of Iscador. Iscador did not change the cellular response to 2 x 10(7) intraperitoneally administered SRBC. The IgM plaque forming cell response was accelerated and followed by an augmentation of the IgG and IgA plaque forming cell response. Evidence is presented that the immunogenic and inflammatory capacities of Iscador contribute to its adjuvant activity. Both micro-organisms and soluble, filter-adherent constituents in Iscador possess adjuvant activity. The relation between the immunostimulating properties of Iscador and its anti-tumour activity is discussed.