A new method is described which permits staining of the histamine in mast cells of embedded tissue. Small specimens of fresh tissue, rapidly frozen in isopentane-liquid nitrogen, are dried in vacuo at-35#{176}C, embedded in paraffin, sectioned and stained with o-phthalaldehyde in ethyl benzene. Mast cell histamine appears bright yellow as viewed under fluorescence microscopy. The procedure is standardized, suitable for a wide variety of tissues and remarkably free of diffusion artifacts. The uncut blocks may be stored for months with no changes in later histamine-staining potential. Alternatively, embedding

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