In order to deduce the unique structure of ox insulin it is necessary to know its molecular weight. In previous papers a value of 12 000 was assumedc since physical measurements suggested that this was the weight of the smallest unit that existed in

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Previous work has shown that insulin consists of two types of polypeptide chains the amino acid sequences of which have been determined by partial hydrolysis experiments (Sanger & Tuppy, 1951 a, b; * Member of the scientific staff of the Medical Research Council.

The rates of decomposition of N-methyl-N′-nitro-N-nitrosoguanidine (NG), N-methyl-N-nitroso-p-toluenesulfonamide (MNTS), N-methyl-N-nitrosourea, and N-methyl-N-nitrosourethane in dilute aqueous solutions at pH levels ranging from 2 to 9 are tabulated. There are considerable differences in the breakdown rates of these compounds. At low pH, MNTS is destroyed several times faster than any of the other compounds, while at pH 7, MNTS is about 100 times as stable as methylnitrosourea. Rates of photochemical destruction of NG and MNTS are also given. Some of the breakdown products have been identified. These data are used to provide tentative explanations for some apparently anomalous biological results.

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