None of the methods currently available can detect the small numbers of active renin (AR) molecules present in plasma. Among seven monoclonal antibodies (Ab), two Abs were selected which did not recognize the same epitope and could be used in a sandwich assay. The first monoclonal Ab, 3E8, binds soluble renin (B 50% = 1 x 10(-10) mol/l) and does not inhibit its enzymatic activity. It was coupled to magnetic beads (Magnogel) and was used to trap both active and inactive renin from 250 microliters plasma. The second Ab, 4G1, binds renin (B 50% = 3.5 x 10(-10) mol/l), inhibits its enzymatic activity, and recognizes inactive renin less than AR. It was iodinated and used to detect AR trapped on Magnogel by the first Ab during a 4-h incubation. The assay can detect 16 pg/ml in human plasma and is highly reproducible. The AR level of 15 normotensive subjects, aged 20-45 years, in an upright posture and on a normal sodium intake, was found to be 41 +/- 18 pg/ml (MRC renin standard). The plasmas were trypsin-activated and their total renin levels were measured with the same pair of monoclonal Abs. The mean value of 286 +/- 142 pg/ml is similar to the value obtained by other assay systems which measure total renin with Abs recognizing both active and inactive renin. The direct measurement of AR provides a convenient and standardized method, since the production of the two monoclonal Abs is unlimited.(ABSTRACT TRUNCATED AT 250 WORDS)