1. Three minute fixation in formol vapor at 44 C, followed by 15 minute washing proved to be the most satisfactory fixation procedure for both "acid" and "alkaline" phosphatase technics as applied to bone marrow smears.
2. For both technics a relation between staining intensity and cellular richness was found.
3. The reaction of normal human bone marrow cells to both phosphatase technics is described. Both are predominantly nuclear in location. Nuclear pattern approached that observed with common staining methods and Feulgen’s reaction. Cytoplasmic reaction was nearly negative. Nonspecific and specific granules do not stain after the "alkaline" technic. Nonspecific granules are negative for "acid" phosphatase, while specific neutrophilic are variable, and eosinophilic, constantly positive. Nucleoli are negative after the "acid" technic, being positive for the "alkaline" enzyme. Mitotic chromosomes are positive for both technics. "Acid" phosphatase reaction in cytoplasmic zones of lymphocytes, erythroblasts, plasmacytes and megakaryocytes, is described.
ACKNOWLEDGMENTS Many points pertaining to this paper were discussed with the late Dr. José Oria, to whom we owe invaluable stimulation and guidance. We are indebted to Prof. W. Buno of Montevideo (Uruguay) for kind advice on the Golgi apparatus of blood cells. We are deeply indebted to Professor O. P. Jones of Buffalo for his painstaking help and advice. We thank Dr. M. A. Jamra (Section of Hematology, Central Laboratory, Hospital das Clinicas da Faculdade de Medicina) and Dr. J. F. Pontes (Section of Clinical Therapeutics, Hospital das Clinicas da Faculdade de Medicina), for part of the material used in this study. We thank Messrs. L. Frankenthal and G. Lonenzini for their aid in the preparation of some of the microphotographs.