The numbers of sperm and polymorphonuclear leukocytes and the motility and progression of the sperm were determined in flushings from the reproductive tract at 7 and 15 hr postinsemination of 28 rabbits in two experimental trials. In Trial 1, isoimmunization with semen as compared with seminal plasma resulted in a trend for fewer sperm to be recovered from the oviducts, significant decreases in motility of sperm from the oviduct, and in progression of sperm recovered from the uterus and a prevention of fertilization (0.0 vs 90%). Insemination of nonimmune rabbits in Trial 2 with semen treated with isoantiserum against semen in comparison to isoantiserum against seminal plasma caused significant decreases in sperm numbers recovered from the oviduct, uterus, and vagina; a decrease in sperm motility in vaginal flushings; a reduced progression of uterine sperm, and an inhibition of fertilization (12.5 vs 95.6%). Leukocyte numbers were influenced by treatment only in vaginal samples of both groups. The range of values for number and motility of sperm recovered from the oviducts overlapped between treated and control groups within experiments suggesting that these factors alone were not responsible for the observed inhibition of fertilization. Correlation coefficients were calculated among the variables in both experimental trials.