Mice immunized with sheep red blood cells produce more direct plaque-forming cells, estimated with the hemolytic plaque-technique, during the primary immune response if they receive in that time 150 mg NaSCN/kg body mass with the drinking-water every day. The thiocyanate quantity of serum in immunized animals is higher than that in non-immunized animals. The titers of antibodies estimated by the hemagglutination test is about 1 titer step higher in thiocyanate-treated animals than in non-thiocyanate-treated animals. That applies also to the secondary immune response, in which course thiocyanate was not supplied. It is possible, that for instance a greater number of memory-cells was produced during the primary immune response in consequence of the thiocyanate application. Studies in similar arrangement with guinea-pigs confirm these statements in certain parameters.