The effect of immunization against oestradiol-17Ã¢ (E2) on fertilization rates and of human chorionic gonadotrophin (hCG) in reducing the proportion of unovulated follicles from super ovulated ewes was examined. Forty South African Mutton Merino ewes were divided into four treatments in a 2 x 2 factorial arrangement with four replications (block). All ewes were synchronized with intravaginal progestagen sponges and super ovulated with a combination of 700 IU of MNSG and 11 mg FSHp. Ewes were actively immunized against E2 six weeks before super ovulation, hCG (1500 IU) was administered intravenously 6 h after the onset of oestrus. Ewes were mated naturally. Recovered embryos were cultured for six days to assess their viability. AT embryo recovery the mean anti-E2 antibody titre (bound 40% of tracer E2) was 217 Â± 251. Immunisation had a positive effect (p < 0.05) on fertility, with 88.8 Â± 2.8% of the recovered ova having been fertilized, as opposed to 79.3 Â± 3.2% for control (no immunization, no hCG) ewes. Immunisation had no significant effect on ovulation rate and did not improve the yield of viable embryos (immunized = 6.7 Â± 1.1; control = 7.5 Â± 1.3) per ewe. Embryos from immunized ewes appeared to possess a reduced viability. A significant (p < 0.05) interaction between immunization and hCG treatment was observed, with hCG increasing (p < 0.01) the percentage of unovulated follicles in control, but not in immunized ewes. It is concluded that (a) immunization of super ovulated ewes against E2 levels in these ewes, (b) immunization against E2 is not a feasible practice for increasing the yield of viable embryos of super ovulated ewes and (c) hCG may have been administered too early after the onset of oestrus, particularly in control ewes.