Pancreatic A cells of several vertebrate species (rat, mouse, pig, hamster, rabbit, dog, guinea pig, hen, pigeon, newt and crucian carp) selectively fluoresced after staining the fresh frozen cryostat sections of the pancreas with 1% o-phthalaldehyde (Carbiochem) in physiologic saline (dissolved at 58°C). The pertinent cells could be identified as A cells in the same sections observed under dark field illumination or phosphotungstic acid-hematoxylin staining. The histamine assay and some experiments using reserpine, compound 48/80 and cobaltous chloride indicated that the fluorogenic material is neither histamine, catecholamine nor tryptamine. This o-phthalaldehyde stain is a simple, rapid and specific method to disclose A cells of the pancreatic islet, although the nature of responsible substance is yet unknown.