A mouse monoclonal antibody (mAb), designated PS-7.6, was previously shown to enhance the activity of porcine growth hormone (pGH) in promoting the growth of hypophysectomized (hypox) rats. Epitope mapping studies indicated that the region recognized by PS-7.6 resided within an amino acid sequence 54-95 of pGH. A peptide corresponding to this sequence was synthesized and found to induce swine antibodies capable of augmenting pGH activity in hypox rats. On the basis of these previous observations, an attempt was made in this study to determine whether or not the peptide pGH(54-95) could be used as a vaccine to elicit antibodies functionally similar to PS-7.6 mAb, thus potentiating the efficacy of endogenous GH in swine. Young pigs (15-20 kg) were immunized with pGH(54-95) that had been conjugated with ovalbumin (OVA) and boosted twice at 4-week intervals. Control animals were similarly immunized with OVA. The weight gains and feed consumption of these animals were closely monitored throughout the trials. A number of carcass parameters were also examined when these animals reached 110-120 kg, at which time they were killed. Results indicated that immunization with peptide significantly accelerated the daily weight gain during the growing phase of growth. However, this effect disappeared during the finishing phase of growth. The failure to prolong the initial growth effect by the peptide immunization apparently correlated with the kinetics of antibody production, because antibodies immunoreactive to the peptide and pGH were detected in these animals after immunization but gradually diminished. This idea was supported by the fact that antibodies obtained from pigs 5 and 9 weeks after the initial immunization potentiated the activity of pGH in hypox rats, whereas antibodies harvested at week 16 did not. Furthermore, carcass evaluation was performed at time of killing and showed that the leaf fat and loin eye muscle were also significantly improved by peptide immunization. Taken together, the present findings suggest that pGH(54-95) peptide can be employed as a potential growth-promoting vaccine to improve the performance of swine.