Histochemically demonstrable ATPase activity has been observed on Ehrlich's ascites tumor cells, human hepatoma, mammary cancer, uterine cancer, and leukemic cells in the liver tissue by the method of Wachstein. Observations proved that the most strong activity of ATPase was seen on the hepatoma tissue whose reaction appear surrounding the whole area of each cell membrane. Moderately positive activity also has been observed on Ehrlich's ascites tumor cells and uterine cancer cells showing the dotted reaction on the cell membrane. Mammary cancer cells and leukemic cell of monocytic type proved to be negative in this reaction. Findings suggestive of a posibility the same as the application of this reaction to the biopsy materials may give some information to know the mother tissue from which cancer originates.