1. Extracts of rabbit kidney contain both active renin and a material which has a scarcely detectable pressor response in the rat at pH 7.4, but which, after adjustment of the pH to 2.5 and then back to 7.4, gives a similar pressor response to renin.

2. The material is eluted from Sephadex G-100 ahead of renin, but after acidification is eluted in the same fractions as active renin. It may be a renin zymogen.

3. The zymogen-like material has a molecular weight of around 54 000, compared with a molecular weight of 37 000 for active rabbit renin.

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