Relaxin is a peptide hormone produced solely by the corpus luteum of pregnant women. Extracts of human pregnancy corpora lutea have relaxin activity in bioassay systems. This activity can be measured in a heterologous porcine radioimmunoassay (RIA). Immunoreactive relaxin was undetectable in the RIA in 51 nonpregnant women. In conception cycles, relaxin immunoactivity is detectable in peripheral blood by the time of the missed menses. Relaxin detection may be used as a pregnancy test. Relaxin may be an important luteal factor, together with progesterone, in early pregnancy maintenance.