Daily levels of plasma progesterone were monitored in 14 pseudopregnant rabbits (controls), eight of which were subsequently immunized with a progesterone-11-bovine-serum albumin conjugate (immunized) and 4 with Freund’s complete adjuvant (adjuvant-treated). Five weeks after the immunization schedule had ended, animals in both the groups were made pseudopregnant again and daily plasma progesterone was determined.

Antibodies to progesterone were not detected in the controls. Plasma progesterone in these animals gradually increased to a peak (11.9 ng/ml) on Day 10 of pseudopregnancy and declined to pre-HCG levels by Day 18. The adjuvant-treated animals showed low but detectable antibody titers. Except for Day 0 of pseudopregnancy, when plasma progesterone levels in these animals were significantly (P<0.01) higher, the pattern in the adjuvant-treated rabbits was similar to that in the controls. At a dilution of 1:100 the mean (± SE) percentage binding of added progesterone-1,2,6,7-³H in the sera of the immunized animals was 57.2 (± 5.2). Plasma progesterone levels in these animals rapidly increased and peaked (55.1 ng/ml) on Day 9 of pseudopregnancy. On Days 0 through 10, 17 and 18 progesterone values in immunized animals were significantly (P<0.05 to 0.01) higher than those in controls on comparable days. It is concluded that immunization of rabbits with a progesterone-protein conjugate leads to abnormally high levels of peripheral progesterone in pseudopregnancy. Further investigation is required to explain this effect.