Summary9 different substrates and 3 different salts of glycerophosphate were used in this histochemical study of phosphomonesterase in smears of rabbit and guinea pig blood and bone marrow. With the use of two substrates, phytin and 1: 2‐methylcyclohexanolphosphate, no reaction was observed in the cells. Phosphomonoesterase with the other substrates was demonstrated distinctly in blood and bone marrow cells. Pictures obtained with the use of several different substrates were somewhat different in a few respects. The substrate specificity of phosphatases was also observed histochemically under phosphomonoesterase. The picture of distribution of enzymatic activity was essentially identical with all substrates, but the reactions were of different character for different substrates either in cytoplasm or in nucleu