An analysis is presented of experimental and clinical data from different authors on the stimulating effect of autohaemotherapy with regard to the immunological reactivity of humans and animals as well as in vitro experiments with lymphocytes. Erythrolysate has been found to exert a more powerful effect than intact erythrocytes. The stimulating effect of autohaemotherapy on both irradiated and non-irradiated animals manifests itself in an increase in resistance to infection (increased LD50 in experimental infection), enhanced production of antibodies to microbial and tissue antigens and activated functioning of cell-mediated immune defence mechanisms. The favourable influences on radioresistance and the antitumour effect of authohaemotherapy are described. Induced desensitization plays an important part in the mechanism of action of autohaemotherapy. The administration of large doses of erythrocytes or of erythrolysate results in immunosuppression. Autohaemotherapy does not cause side effects and is feasible both on an in-and out-patient basis.