A simple method for supravital fluorochroming of blood with acridine orange has been described. The supravital method is of value in distinguishing nucleated cellular elements, reticulocytes and erythrocytic inclusions. Preliminary studies of peripheral blood in leukemia indicate that it may be helpful in differentiating certain abnormal cellular elements. The chemistry of acridine orange with respect to metachromatic color differentiation of DNA and RNA has been discussed. Based on supravital observations of the L. E. cell phenomenon, speculation has been offered as to the site of interaction of the L.E. factor with DNA.