The objective of this experiment was to determine whether passive immunization against a synthetic inhibin-like peptide (alpha-IF) increases FSH secretion and reproductive performance of ewes bred in the spring. Crossbred ewes were assigned to three treatment groups of 10 ewes each. Progesterone-releasing pessaries (CIDR-G) were inserted and left in place for 12 d. The CIDR-G were removed on April 7 (0 h), and three rams were introduced. A single 1.8-mL i.m. injection containing 0, 315-, or 630-reference preparation-2 (RP-2) kU semipurified alpha-IF-antibody (Ab) was given at -48 h. Antibody solution was prepared by ammonium sulfate precipitation and concentration of ovine immune sera that had been generated against alpha-IF-human-alpha-globulin. After the alpha-IF-Ab injection, plasma alpha-IF-Ab titers and FSH concentrations increased within 6 h, peaked 12 to 24 h later, and then decreased. At 12 h after injection, FSH concentrations were increased (P = .05) 28 and 42% over control values in ewes injected with 315- and 630-RP-2 kU alpha-IF-Ab, respectively. All ewes expressed estrus and ovulated, except one control that did not ovulate. Ovulation rate (OR) was increased (P < .0001) by alpha-IF-Ab treatment. Mean OR was 1.0 in control ewes, 1.7 in ewes given 315-RP-2 kU, and 2.5 in ewes injected with 630-RP-2 kU. Fertility (ewes lambing/ewes exposed) from breeding at the induced-synchronized estrus tended (P = .09) to be greater for ewes injected with 630-RP-2 kU alpha-IF-Ab.(ABSTRACT TRUNCATED AT 250 WORDS)