A methanolic solution of the oxazine textile dye, C. I. basic blue 122, followed by an aqueous alkaline solution of the oxazine dye, C. I. basic blue 141, and a brief rinse in an acetate buffer at pH 3.45 produces intense black staining of eosinophil granules. This staining was selective for eosinophils while other types of peripheral blood leukocytes showed little if any staining under the same conditions. This staining procedure may be useful for detecting eosinophils in samples of blood, bone marrow, or urine when eosinophiluria results from interstitial nephritis.