1. Several commonly used preparations of human renin, including the International Reference Preparation (Renin Standard 68/356), were examined for the presence of contaminating proteinase activity by using a 14C-glycinated bovine haemoglobin substrate assay.
2. All of the human renin preparations tested cleaved haemoglobin even in the presence of di-isopropylfluorophosphate and ethylenediaminetetra-acetate (EDTA). For a given amount of renin activity, varying amounts of proteinase activity were seen. The pH optimum also varied between preparations.
3. Small peptide inhibitors of human renin were not able to inhibit the proteinase activity. Furthermore a diazoacyl reagent and pepstatin, both potent inhibitors of aspartic acid-active site proteinases, were only partially inhibitory.
4. These and other observations suggest that the proteinase activity of the human renin preparations is not due to renin itself, but to contaminating proteinases of different types. Since these enzymes may produce angiotensin I or peptides which may interfere in renin assays, crude preparations of renin which contain proteinase activity, including the International Reference Preparation, should be used with caution or replaced by proteinase-free human renin which can be easily prepared by use of suitable affinity chromatography.