Summary. The biotin—avidin immunoperoxidase staining method and antisera against highly purified porcine relaxin were utilized to localize relaxin in the male genital tract. Formalin-fixed tissue specimens from the prostate gland, the testis, epididymis, vas deferens and the seminal vesicle were studied. Specific relaxin immunoactivity was seen in the glandular epithelium of the prostate and also in the glandular epithelium of the seminal vesicle and in the ampullary part of the vas deferens. The testis and epididymis were negative.