Selective fluorescence reaction of indigocarmine stained eosinophil leucocyte granules induced by alkaline reduction of the bound dye to its leuco derivative.

After staining of mammalian blood smears with indigocarmine, eosinophil granules were the unique cell components which stained deeply blue under bright field illumination. Treatment of stained smears with the reducing agent sodium borohydride completely abolished this colour reaction, while under violet-blue exciting light eosinophil granules appeared with bright green fluorescence. Other reducing agents proved less suitable. Spectral analysis of indigocarmine solutions reduced by sodium borohydride showed an emission peak at lambda = 528 nm and confirmed the microscopic observations. These results indicate that the treatment of indigocarmine stained structures with alkaline solutions of strong reducing agents converts the bound dye into its reduced and highly fluorescent leuco derivative.