A new type of biological particle, isolated from the marine dinoflagellate Gonyaulax polyedra, has been partially purified and characterized. When the pH is lowered, the particle emits light in vitro in a fashion closely mimicking the flash of the living cell, and it is referred to as a scintillon (flashing unit). Scintillons are obtained by breaking the cells in buffer at pH 8.2 and purifying by differential and sucrose density gradient centrifugation. The particle has a density of about 1.23 g cc-1, and activity is quantitatively correlated with the number of crystal-like birhombohedral structures. These have been found to contain guanine, but since the density of authentic guanine is about 1.73 g cc-1, the scintillon is believed to comprise additional but as yet unidentified components. The properties of the scintillon and the effects of various physical and chemical treatments are described. The reasons for believing that this particle is responsible for the flash of the intact cell are discussed.

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