Spermatozoa were recovered from the uteri and oviducts of mated does between 0 and 14 h post coitum (p.c.). The proportion of spermatozoa binding IgG before and after treatment with normal rabbit serum was assessed by immunofluorescence microscopy. Most spermatozoa recovered from the oviduct and from the uterus < 3 h p.c. did not bind IgG, in contrast to those of the ejaculate which did, and those from the uterus 4--10 h p.c. which were already coated with IgG when recovered. Spermatozoa transferred at 2 h p.c. to a recipient doe did not bind IgG 4 h later although a 6 h sample of the donor's spermatozoa did so; only approximately 150 of these spermatozoa were needed for all fertilized eggs/doe, whereas 10(4)--10(5) ejaculated spermatozoa were required when similarly inseminated. We suggest that the small numbers of spermatozoa that do not coat with IgG and are first through the cervix are those which normally fertilize. Whether they are are separate population in the ejaculate is uncertain.