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These investigations originated in studying the basis of a method for microbiological assay of nicotinic acid (McIlwain & Stanley, 1948). The method was that in which acid produced from glucose by Lactobacillus arabinosus 17-5 is titrated with alkali after a period of incubation of about 3 days. The assay depends on the empirical observation that the quantity of titratable acid produced during incubation is dependent on the quantity of added nicotinic acid. It was found (McIlwain, 1948) that this relationship existed because of changes which took place in nicotinic acid during the assay; the compound was first assimilated to, and later lost from, the cells. Its loss was found to be conditioned by its functioning as a catalyst, and was such that nicotinic acid exhibited a defined catalytic capacity, measured by the ratio: titratable acid formed/nicotinic acid lost (inmol./mol.). Values ofthe catalytic capacity (about 5 x 105) were relatively independent of the intensity with which nicotinic acid derivatives were functioning as& catalysts, which was expressed as a catalytic activity (in mol. titratable acid/hr./mol. nicotinic acid; values rangedfrom 5 to 15 x 103 hr.-1). Such observations suggested the need for more detailed investigations ofthe form in which nicotinic acid derivatives existed in the bacterial cells. We have now examined the nicotinic acid of cells of Lb. arabinosus, and found it to be largely in the form of cozymase. Lb. arabinosus, in marked distinction from a number of other organisms, reacted only slowly with added cozymase; this, and our interest in the practical problems presented by microbiological assay, directed our attention to metabolic changes occurring in the cozymase of the bacterial cells, and these are described below.

The behaviour of Lactobacillus arabinosus towards nicotinic acid and its derivatives