Ten broiler-type chickens at 2 weeks of age were injected daily with 0.5 mi of normal sheep serum while 10 others were similarly injected with 0.5 mi of a sheep anti-IGF-2 serum. Immunization with anti-IGF-2 serum had no significant effect upon body weight gain, on carcass composition, on appetite or food conversion. Liver weight was significantly increased (P < 0.05) in the anti-IGF-2 birds, but there was-no effect on the weight of any other organ measured. The proportion of breast muscle in the carcass of anti-IGF-2 treated birds was significantly lower (P < 0.01) than in the controls and they also had 27% less abdominal fat. In acute studies, anti-IGF-2 administration caused an elevation in plasma GH, but in the longer term neither plasma GH nor plasma T-3 concentrations were significantly affected by immunization against IGF-2. These results suggest that circulating IGF-2 is not a major regulator of overall somatic growth in chickens, but may have an effect on muscle and fat deposition. (C) 1997 Elsevier Science Inc.