The reinsemination of human oocytes that failed to show any evidence of fertilization 12 to 14 hours after insemination was examined to determine whether this would increase success of in vitro fertilization or be of use in determining the fertilizing capacity of the husband's spermatozoa. Reinsemination with spermatozoa of a donor of known fertility resulted in a significantly higher fertilization rate than reinsemination with the husband's spermatozoa. However, the rate of the spontaneously delayed fertilization in oocytes that were not reinseminated was not significantly different from fertilization rates following reinsemination with either the donor's or the husband's spermatozoa. One pregnancy that miscarried at 12 weeks' gestation was obtained from reinseminated oocytes, and none was obtained from oocytes that were spontaneously delayed in fertilization. It is concluded that at present neither reinsemination nor embryo replacement in utero of embryos resulting from delayed fertilization is useful for human in vitro fertilization.