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As yet no entirely satisfactory microbiological method for the assay of cystine has been reported. Assays are described, using Lactobacillus arubinosus, by Shankman, Dunn & Rubin (1943) and by Barton-Wright (1946), but the absolute requirement of cystine by L. arabinosus has not been confirmed by Riesen, Spengler, Robblee, Hankes & Elvehjem (1947) or by work in this laboratory (unpublished data). With oxidized-peptone medium, Lyman, Moseley, Wood & Hale (1947) found that, though smooth and regular standard curves could be obtained for methionine, tyrosine and tryptophan on media in which most of the amino-acids required were supplied by oxidized- peptone, standard curves for cystine using the same organism (L. casei €, L. helveticus) proved to be irregular and inconsistent. Similar conclusions about the non-reproducibility of standard curves for cystine with this organism were reached by Shankman et al. (1943). Dunn, Shankman, Camien, Frank1 & Rockland (1944) stated that cystine could be determined by Leuconostoc mesenteroides P-60, and Riesen et al. (1947) concluded that this organism is the most suitable for the determination of this amino-acid. Assays of cystine carried out in this laboratory with Leuconostoc mesen- teroides P-SO on oxidized-peptone medium or a known mixture of amino-acids yielded standard curves which varied in range of titre from assay to assay. No such effects were observed when methionine or tyrosine were assayed using the same organism and similar media, and the assay range, as shown by higher titres, was greater in either case than in assays of cystine under the same conditions. With regard to media for cystine assays, oxidized-peptone is to be preferred since blanks are usually lower than those obtained on chemically defined media (amino-acid mixtures). Riesen et al. (1947), also, have demon- strated the superiority of oxidized-peptone medium for the assay of cystine, using several organisms for which cystine is said to be essential. There is a further complication due to an apparent toxic effect, characterized by a slight break in the assay curve but also shown by a clumping of the bacteria. Fig. 1 shows a standard curve for cystine with rather better than average titres, compared with a normal standard curve for methionine. The same oxidized-peptone medium was used in each case, with the appropriate omissions of methionine and cystine respectively. The clumping of the bacteria results in

The Toxicity of Small Concentrations of Cystine to Acid-producing Bacteria