The effect of dietary protein supply on muscle development and circulating concentrations of insulin-like growth factors (IGF)-I and -II was examined in chickens selected for increased breast yield and decreased fatness (quality, QL) and in its control line (CL). CL and QL chickens were fed isoenergetic diets containing 121.5 or 215.8 g CP/kg during a 12-d period; comparisons were performed at 33 d of age. Birds given the high protein diet grew faster, ate less feed, had lower feed conversion ratio (FCR), and higher muscle weights than their counterparts given the low protein diet. The muscle weight response to protein supply differed between muscles in both lines, with pectoralis major appearing more sensitive than sartorius. The response of the gastrocnemius muscle depended on the line. Selection for carcass quality increased (P < 0.01) body weight, growth rate, feed intake, pectoralis major and sartorius muscle weights, and pectoralis major muscle proportion. There was, however, no line difference in FCR or in sartorius muscle proportion. The weight and proportion of the gastrocnemius muscle were higher (P < 0.05) in the QL than the CL chickens on the high protein diet, but there was no line difference for the low protein diet. Plasma levels of IGF-I, and to a lesser extent IGF-II, were lower (P < 0.01) in protein-restricted chickens. No difference in circulating IGF-II was observed between the lines. Concentrations of IGF-I were higher (P < 0.05) in QL than CL chickens, which may contribute to improved body composition for this genotype.