An experiment was performed to evaluate the effects of active immunization of heifers against estradiol on feedlot performance (growth and efficiency), carcass characteristics, and reproductive functions. Seventy-two crossbred heifers were divided into four equal treatment groups consisting of controls, ovariectomized heifers, and heifers actively immunized against keyhole limpet-estradiol antigen and bovine serum albumin-estradiol antigen. Heifers were fed ad libitum for 170 days. Both groups of heifers immunized against estradiol had higher (P less than 0.05) average daily gains than controls. Heifers immunized against bovine serum albumin-estradiol had increased feed efficiency (P less than 0.05) over controls. Ovariectomized heifers did not perform at levels sufficient to compensate for the initial setback from surgery. No differences were noted in carcass grade, quality, or concentration of water, fat, or protein. Uterine weights were increased in estrogen-immunized animals but were not significantly greater than controls. Ovarian weights and numbers of large follicles (greater than 9 mm diam) in immunized animals were significantly greater than in controls. Twenty-eight percent of the animals (n = 5) in the bovine serum albumin-estradiol-immunized group had cystic follicles (greater than 20 mm diam) and 50% (n = 9) of this group had no detectable corpus luteum. Low titer (1:100) systemic binding of estrogens may act as a steroid reservoir in which systemic estrogen clearance is decreased and availability to target tissues is increased.