Partially-purified inactive renins from human plasma and kidney seem to be identical in most respects except for apparent molecular size. To evaluate this difference, we determined apparent molecular weights by gel filtration with internal radiolabeled standards, using trypsin activation in the presence of benzamidine and albumin to provide reproducible detection. While there was a suggestion of a shoulder in the 50,000-dalton region of the plasma inactive renin peak, the major form (56,000) was consistently larger than that of renal inactive renin (50,000), confirming our previous observation. Since both renal and plasma inactive renins appear to be glycoproteins, based on their ability to bind to concanavalin A-Sepharose, it is possible that differences in carbohydrate composition might contribute to this discrepancy in gel filtration behavior. The striking similarity of these substances in all other respects, including inhibition of the activated forms by monospecific antirenin antibodies, makes it unlikely that they differ in primary structure.