1. Body composition was determined in 9-week-old chickens sampled from four lines selected either for increased body-weight gain (line W), for increased food consumption (line F), for decreased food conversion ratio (line E), or at random (line C), after three, four and five generations of selection.2. After five generations of selection the proportions of carcass water (678 g/kg) and protein (187 g/kg) were highest in line E and lowest (636, 180 g/kg respectively) in line F. The proportion of fat was lowest (83 g/kg) in line E and highest (134 g/kg) in line F.3. Lines W and C were similar and intermediate to lines E and F for all the carcass constituents.4. There was more fat in females (115 vs 98 g/kg) and less protein (177 vs 190 g/kg) and water (652 vs 665 g/kg) than in males.5. Line differences in carcass composition were not appreciably altered if birds were killed at equal weight rather than equal age.

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