Relaxin immunoreactivity has been found in samples of human follicular fluid collected from artificially stimulated pre-ovulatory follicles. The crude extract caused a reduction in the height of the contractions in a rat uterine strip bioassay. The reactive material eluted from Sephadex G50 in two major peaks. The first contained ̃ 60% of the immunoreactivity and had an elution position corresponding to that of porcine relaxin, indicating a mol. wt of ̃ 6000 daltons. The second peak was of a lower mol. wt, but its exact size and significance are unknown. A possible role for relaxin in the process of follicular rupture is suggested. © 1986 IRL Press Limited.