Relaxin is a protein hormone composed of two amino acid chains. The role played by relaxin in human pregnancy and parturition is unclear. Its use and involvement as a cervical ripening agent has been debated since the 1950s. Because the main source of human relaxin is the corpus luteum of pregnancy much of the early work on induction of labour has focused on porcine or bovine preparations. With the advent of DNA recombinant technology human relaxin has become available for evaluation. Relaxin is thought to have a promoting effect on cervical ripening. Due to a possible inhibitory effect on human myometrial activity, relaxin may not be associated with the concomitant increase in the rate of uterine hyperstimulation seen with other induction agents. This is one of a series of reviews of methods of cervical ripening and labour induction using a standardised methodology.