(1) A method is described for the supravital staining of leukocytes with neotetrazolium chloride, using EDTA as the anticoagulant.

(2) The reaction, as demonstrated by this method, is not dependent on any particular substrate and cannot be regarded as a function of specific dehydrogenase. It permits, however, the direct visualization of endogenous reductase activity.

(3) The results, expressed as the percentage of leukocytes stained, show a relation to leukocytosis. Readings below 20 per cent are infrequently obtained with nonleukemic specimens when the leukocyte count exceeds 13,000/cu.mm. In leukemia low ratings are the rule, even in the presence of a high white blood count. In other diseases, including malignant lymphomas, the number of stained cells roughly parallels the elevation of the white blood count.

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