The pioneering work of Cohen, S., Taylor J.M., Murakami, K., Michelakis, A.M. and Inagami, T. ((1972) Biochemistry 11 4286-4293) with isolation of submaxillary renin and the first direct radioimmunoassay for renin subsequently described by Michelakis A.M. Yoshida H., Menzie, J., Murakami, K. and Inagami, T ((1974) Endocrinology 94, 1101-1105) was reinvestigated and confirmed. In addition a detailed evaluation of the assay is described by characterization of the immunoreactivity of the labelled renin and demonstration of immunological identity between plasma renin and purified renin standards. Assay and equilibrium conditions were characterized and optimized and the association constant for the antibodies determined to be of the order of 10(11)1/mol. The validity of the direct assay for renin was further investigated by a comparison with another radioimmunoassay for renin; the antibody trapping method, which measures the enzymatic activity of renin in plasma. The correlation coefficient is high (0.91) but does not exclude that the direct assay measures a hypothetical prorenin in plasma as well.