Relaxin was purified from human term placentas. A fraction having a molecular weight of approximately 6000 and a specific activity of 11.9 U/mg was eluted from a Bio-Gel P-30 column. This fraction inhibited uterine contractions and produced a linear response to log doses of relaxin in the mouse interpubic ligament bioassay. Electrofocusing of the active Bio-Gel P-30 fraction separated a protein peak with a biological activity of 45 U/mg protein and an isoelectric point of pH 11.4. The electrofocused human relaxin fraction showed cross-reactivity with antiserum produced against highly purified porcine relaxin and formed a single precipitin line with no spurring when compared to purified porcine relaxin in double immunodiffusion assays. Human relaxin was inactivated by the reducing agent dithiothreitol, indicating that disulfide bonds are essential to the biological activity of human placental relaxin. Immunohistochemistry demonstrated that relaxin was located in cells of the basal plate and septae of the placenta.