The effects of storage on stability of six types of commercial insulins have been studied at 5°, 25°, 37°, and 50° C. for periods up to thirty-six months. Samples of insulin from thirty-two batches at concentrations of 40 and 80 U. per ml. were used.

Use of the radioimmunoassay made it practical to perform the large number of potency determinations required to obtain the data in the present report. These data show that no appreciable loss of potency occurred in any of the insulins during thirty months of storage at 5° C. At higher temperatures extent of potency loss was influenced by length of storage and type of insulin. In general, all modified insulins studied were more stable than Regular insulin. Under the same conditions Protamine Zinc and NPH insulins were more stable than the Lente insulins.

The biological reaction (duration of action) of the modified insulins which had beenstored for twenty-four months at 5°C. were compared with freshly prepared samples. No significant changes in the response curves characteristic for each insulin type were observed. Potency determination by the U.S.P. rabbit assay compared satisfactorily with data obtained by radioimmunoassay

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