Pinacyanol1 is an excellent, almost permanent and very selective stain for mitochondria in supra-vital blood preparations. It may be used alone or in combination with neutral red for the study of fresh blood, bone marrow, spleen or lymph node smears. One drop of a 0.1% solution of pinacyanol in absolute alcohol, per cubic centimeter of the same solvent, is a satisfactory dilution for making stain films for blood with an average concentration of cells.