Endogenous and succinic dehydrogenase activity was demonstrated in the living cells of normal human blood and bone marrow using a buffered nitro BT-succinate incubating solution. With this technique dehydrogenase activity was localized primarily in the granular leukocytes and the sites of enzymatic activity appeared to be non-mitochondrial. The addition of a non-ionic surface active agent to the incubating solution resulted in marked differences in the cellular and intracellular localization of dehydrogenase activity. With this method it was possible to demonstrate dehydrogenase activity in the mitochondria of most of the formed elements of the blood and bone marrow, including developing granulocytes and erythroid cells, agranulocytes, and blood platelets. Mature erythrocytes also exhibited a minimal dehydrogenase reaction with this procedure. This investigation indicated that in order adequately to demonstrate and evaluate dehydrogenase activity in the cells of the blood and bone marrow it was necessary to have increased cellular and mitochondrial permeability, as well as partially viable cells with an intact dehydrogenase system.