Heifers were immunized with rabbit semen. The immune sera (IS) from them, in high concentrations, caused tail-tail agglutination of rabbit sperm in contrast to a head-head type produced by normal sera (NS). Diluted rabbit semen (same concentration for each trial) was mixed with NS and IS approximately 15 min before being used for insemination.

Normal serum did not affect fertilization. High concentrations of IS prevented fertilization but the fertilization rate increased as lower concentrations were used (173 females studied).

Embryo survival percentage was estimated in the NS and IS groups by a comparison of the number of fertilized ova in a random sample of females killed at 28 hours after insemination with the number of surviving fetuses in a comparable group of females at 28 days. The average embryo survival percentage for 58 IS females was 38.2 compared to 53.1 for 59 NS females. The loss of whole litters was much more common in the IS group (23 females out of 58) than it was in the NS group (3 females out of 59). Resorbing embryos or placental sites were found in 17 of the 23 IS females. A significantly greater amount (P < 0.005) of embryonic death occurred in the IS group.

The mechanism by which the antibodies caused fertilization failure and embryonic death is not known. The effect on fertility could not be explained on the basis of the gross amount of sperm agglutination or any marked interference with sperm motility.

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