AbstractThe effects on mouse embryo development in vivo of varying the numbers of spermatozoa used in artificial inseminations was studied. The two criteria used in the evaluation of the progress of embryo development were 1) ability to reach the two‐cell stage and 2) success of development from the two‐cell stage through implantation. A 44% reduction in the yield of two‐cell embryos and a 67% reduction in the number of implants was observed when C3HeB/FeJ females were inseminated with one‐twentieth the number of spermatozoa estimated to be present in a typical ejaculate. The reduction in the yield of two‐cell embryos was substantially reversed by a second insemination of a large number of heat‐inactivated spermatozoa 12 hr after the first insemination. The sperm‐dependent reduction in development from the two‐cell stage through implantation was prevented only by normal viable (unheated) spermatozoa. These results were rationalized by the hypothesis that in female C3HeB/FeJ mice spermatozoa serve physiological functions beyond the fertilization of ova and that spermatozoa may act to foster early embryo development through modulation of the environments embryos experience as they move through the rep