Adenosine triphosphatase activity has been localized in human blood cells by combined histochemical and electron microscopic technics. Deposits of lead phosphate, indicative of enzymic activity, were observed on the surface membranes of erythrocytes, lymphocytes and platelets. Intracellular ATPase activity was found in lymphocyte mitochondria and nucleoli. Blood platelets contained organelles in their hyaloplasm with dense accumulations of the enzyme reaction product. The distribution of lead phosphate in platelet organelles suggested that they were mitochondria, but some platelet granules may also contain ATPase activity.