A differential nucleic acid stain for peripheral blood and marrow smears using methyl-green and pyronin Y and Carnoy’s solution as a fixative is described. Characteristics of the various blood cells are quite distinct so that identification is readily accomplished.

If pyroninophilia is assumed to be a measure of RNA, morphologically identical and mature leukocytes are found to vary in the quantity of cytoplasmic RNA. Since cellular immaturity is associated with increased RNA, this must be an indication of the varying "age" of white blood cells in the peripheral blood. With the use of this stain a much more dynamic picture of white cell physiology is obtained than with other more conventional stains.

The quantity of cytoplasmic RNA in peripheral blood leukocytes, as measured by a scoring technic, was found to be increased as compared to normal in most infections, leukemias, pernicious anemia in relapse, and a variety of other conditions. With treatment, the score fell to normal levels in pernicious anemia and infections.

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