Rabbit and mouse spermatozoa from male and female tracts have been examined for their species-antigenic surface character, and for adherent antibodies, by double immunofluorescence techniques. Mouse spermatozoa from the ductus deferens showed an area over the acrosome which was positive to anti-mouse serum that had been absorbed with some male mouse somatic tissues including blood, but those from the uterus and oviduct were not stained. Spermatozoa from the uterus were shown to have an antibody coat on the acrosome, with anti-mouse IgG, but those from the ductus deferens and oviduct did not. Rabbit spermatozoa were more variable but their activity was similar: ejaculated spermatozoa sometimes already had antibody of male origin; the majority of the spermatozoa arriving early in the uterus were coated, but in general those that attained the oviducts were not coated. The results are interpreted as evidence for selection by the female tract of a small antigenically different population; the majority of spermatozoa are rejected and/or destroyed.