A sandwich e.l.i.s.a. method was developed to examine the distribution of lactoferrin in mouse reproductive tract. The lactoferrin concentration was found to be much higher in oviduct, uterus, vagina, vas deferens or epididymis than in serum, but the concentration in ovary, testis, seminal vesicle, prostate or coagulating gland was comparable with that in serum. The existence of lactoferrin in male sexual organs was confirmed by Western-blot analyses for tissue proteins. Lactoferrin in male sexual organs was shown to have a molecular mass similar to that of the deglycosylated form of lactoferrin purified from mouse uterine luminal fluid. Northern-blot analyses for total RNA prepared from male sexual organs indicated that only epididymis contained the lactoferrin mRNA. The lactoferrin mRNA was found in the prepubertal period and increased with the growth of epididymis. The mRNA level in prepubertal epididymis could be stimulated by 17 beta-oestradiol, but was not influenced by testosterone.