The specificities of two radioimmunoassays (RIA) for relaxin, based upon crude porcine relaxin (NIH-R-P1; RIA I) and a highly purified porcine relaxin (RIA II) have been studied concurrently using purified hormones and plasma samples. A labelled fraction, selected from radio-iodinated NIH-R-P1 and used in that RIA, was also bound to antiserum raised to the highly purified relaxin. Hence a third RIA was possible in which both the crude and the purified relaxins inhibited in the ng/ml range. Porcine insulin and the connecting peptide of porcine proinsulin did not inhibit any of the assay systems whereas porcine proinsulin did inhibit in each assay at the microgram/ml range. Concurrent measurements by assays I and II have been made in sheep plasma obtained during both delivery of the lamb and suckling. The peak values obtained by assays I and II are 3 and 6 min out of phase during suckling and delivery respectively; the NIH-R-P1 relaxin immunoactivity appearing first. The plasma inhibition curves of both appear to be the sum of individual contributions from relaxin and relaxin-like peptides, such as prorelaxin and its fragments, as seen by different antisera. Both assays, however, give qualitatively similar indices of relaxin immunoactivity. The RIA developed for the more purified peptide would be expected to yield a better quantitative estimate of relaxin secretion but this, like specificity, cannot be shown absolutely.