Relaxin (RLX) has been purified from the ovary of the pregnant pig and rat but not from human tissues. Our study shows that tissue extracts and incubation media of in vitro culture of human decidua contain a substance with relaxin bioactivities: the inhibition of spontaneous uterine contractions and the elongation of the interpubic ligament. After chromatography on Sephadex G-50 the bioactivity was retained in a protein fraction of approximately 6000 daltons mol wt. The yield of RLX from decidua was 15--33.5 GPU/g fresh tissue. This opens the possibility of the isolation and purification of RLX in the human species.