1. The response of various tissue cultured cells to homologous and heterologous blood group specific sub tances has been studied using cytological criteria and chromatography of isolated cell mucopolysaccharides. 2. Evidence is presented that blood group substances inhibit migration rate of explanted rat bone marrow cells and human leucocytes, proliferation of HeLa, Detroit 6 and human embryonal skin cells, growth and mitotic rate of chicken fibroblasts. The blood group substances produce visible signs of cell damage and reversible changes in the chromosome stem line number in HeLa-cells. 3. Capability of HeLa-, Detroit 6 and skin cells to bind heterologous and homologous blood group substances when exposed to them has been visualized by chromatography of cell mucopolysaccharide preparation where a greater occurrence of sugar compounds originating in blood group substances added could be detected after exposure. 4. The cytotropic action appears to be produced by the mere macromolecular nature of blood group substances irrespective of serological group specifity but preparations of human origin were somewhat more effective than the animal ones. 5. Some general aspects and deductions concerning the incorporation and mode of action of blood group substances are discussed. The response of various tissue cultured cells to homologous and heterologous blood group specific sub tances has been studied using cytological criteria and chromatography of isolated cell mucopolysaccharides. Evidence is presented that blood group substances inhibit migration rate of explanted rat bone marrow cells and human leucocytes, proliferation of HeLa, Detroit 6 and human embryonal skin cells, growth and mitotic rate of chicken fibroblasts. The blood group substances produce visible signs of cell damage and reversible changes in the chromosome stem line number in HeLa-cells. Capability of HeLa-, Detroit 6 and skin cells to bind heterologous and homologous blood group substances when exposed to them has been visualized by chromatography of cell mucopolysaccharide preparation where a greater occurrence of sugar compounds originating in blood group substances added could be detected after exposure. The cytotropic action appears to be produced by the mere macromolecular nature of blood group substances irrespective of serological group specifity but preparations of human origin were somewhat more effective than the animal ones. Some general aspects and deductions concerning the incorporation and mode of action of blood group substances are discussed.