A histochemical study on alkaline phosphatase, acid phosphatase, β-esterase, aminopeptidase and β-glucuronidase has been caried out to clarify the relationship between the function and morphology of the human breast tumors; mastopathia: 63, fibroadenoma: 8, gynecomastia: 7 and cancer: 34. The surgically removed tumors were cut at 20μ in -20℃ cryostat, then the sections were incuvated in each histochemical reaction mixture. For the histochemical demonstration of hydrolytic enzymes, azo-coupling methods were employed, i.e., alkaline phosphatase, acid phosphatase and β-esterase were demonstrated by the methods described by Pearce, (60)) β-glucuronidase by Seligman et al, (61)) and aminopeptidase by Nachlas et al. (62)) Alkaline phosphatase activity was the strongest in capillary vessels and myoepithelium and moderate in the normal lobulus and ducts. This enzyme activity increased in proliferating area, but in precancerous area and in cancer it decreased markedly. Acid phosphtase activity was usually low in normal glands and not prominently decreased in cancer than that of alkaline-phosphatase. β-Esterase activity was moderate in breast tumors but increased in periluminal region and necrotic tissue. Aminopeptidase activity was moderate in normal glands but strong in proliferating area. This enzyme activity was elevated in precancerosis, but decreased in cancer cells. β-Glucuronidase activity was increased in proliferating area and precancerous area, but slightly decreased in cancer. This enzyme activity was the highest in duct papillomatosis.