Abstract: A radioimmunoassay method for the in vitro determination of the renin activity of an unknown plasma sample. In this method an unknown generation sample is provided by mixing the unknown plasma sample with a generation buffer solution and an inhibitor for enzymes which convert angiotensin I to other substances. The unknown generation sample is incubated to generate angiotensin I therein by action of renin upon angiotensinogen, thereby producing a generated unknown sample. A generated unknown radioimmunoassay reaction mixture is provided by mixing the generated unknown plasma sample with a predetermined amount of radioactively labeled angiotensin I, a predetermined amount of an antibody for angiotensin I, and an amount of an assay buffer solution sufficient to provide in the unknown reaction mixture renin and angiotensinogen concentrations at which there is no substantial angiotensin I generation at the temperatures to which the generated unknown radioimmunoassay reaction mixture is subsequently exposed.