SU1MMARY: The cultural characters of a strain of Lactobacillus arabinosus and L. helveticus used in the microbiological assay of riboflavin and nicotinic acid proved to be typical, excepting that neither strain fermented xylose and the strain of L. arabinosus fermented rhamnose but not raffinose. Neither produced catalase, so that a strongly positive catalase test in these cultures indicates probable contamination by common air-borne micro-organisms. Modifications of the medium to achieve maximum acid production were investigated by altering, omitting or adding various constituents. A modified medium was adopted allowing a slightly higher acid production. In this medium the maximum acid production was obtained in the presence of natural substances like peptone. The maximal acid production obtainable with nicotinic acid alone was slightly lower, suggesting an additional specific factor in peptone. No confirmatory evidence, however, could be obtained of its existence. Linoleic acid in a concentration of 640 pg./lO ml. medium depressed the acid production, and its action was antagonized by cholesterol. Other fatty acids and lipids had no effect. A good reproducibility and a small coefficient of variation was found between tubes at various levels of nicotinic acid within any one assay, but between separate assays the variation was high (coefficient of variation = 12-19 yo). Since Snell & Wright (1 941) published their original microbiological procedure for the estimation of nicotinic acid, it has been modified by a number of workers (Krehl, Strong & Elvehjem, 1943; Barton-Wright, 1944, 1945; Sarett, Pederson & Cheldelin, 1945). Experience in the assay of riboflavin with Lactobacillus helveticus showed that, by the use of an improved medium, a higher acid production and an increased reproducibility could be obtained (Kodicek, 1948; Pepper, 1947). An attempt was made to improve conditions likewise for the nicotinic acid assay, with L. arabinosus as the test organism. EXPERIMENTAL Cultural characteristics of Lactobacillus arabinosus and Lactobacillus helveticus The characteristics and biochemical reactions of L. arabinosus were compared with those of L. helveticus. Snell's original strain L. arabinosus, 17-5 (Snell & Wright, 1941) was obtained from Dr Snell in 1944. After monthly subculture on yeast-water glucose agar slopes, and storage of the tubes at 4O, it was found in 1945 to be unsuitable for the assay of nicotinic acid. The bacteria had lost ,their sensitivity; their response to graded doses of nicotinic acid was very erratic, and the agreement between tubes at each level of addition was poor. A fresh culture kindly supplied by Dr Barton-Wright was investigated. It proved to be thep-aminobeneoic acid mutant described by Snell (see Shankman, 1943). Lewis also described this mutant strain (see Pennington, 1946) and