The relaxin-like factor is a hormone

The relaxin-like factor (RLF) circulates in the bloodstream of humans, inte racts with a membrane protein with all the characteristics of ligand-recept or binding, and must therefore be considered a hormone by definition. The p olyclonal antibody raised against synthetic human RLF showed no crossreacti vity to other structurally related hormones, like insulin and relaxin. The sensitivity of this assay (ED50 at 100 pM) allowed the direct measurement o f RLF concentrations in serum. The highest levels were detected in the seru m of postpuberty males (190 pM), whereas in females and children, the RLF c oncentration was one order of magnitude lower.