The experiment was carried out on ISA 215 and Ross 308 broilers. Within each genotype, two hundred 21-day-old chickens were allocated to 4 groups of 50 birds (5 replicates of 5 males and 5 females per group). From days 22 to 42 of life the birds of both crosses were given two barley-based diets containing a high proportion of rapeseed meal and a mix of legume seeds. The formulation of the diet was based on total (TAA) or digestible amino acids (DAA), and diets were prepared with or without supplementing a commercial enzyme preparation containing β-glucanase, cellulase, xylanase, α-amylase and protease. Compared with ISA, Ross birds had lower (P<0.01) body weight (BW) gains, but greater breast muscle yield (151 vs 162 g/kg BW; P<0.01) and lower fat content in this muscle (10.6 vs 9.9 g/kg; P<0.01). In ISA broilers, there was an increase in the level of muscle fat in the birds receiving the DAA diet. The significant interaction showed that the level of C18:0 acid increased in ISA females fed the DAA diet. The enzyme supplement increased (P≤0.05) the crude protein (CP) content and reduced (P≤0.01) the proportion of fat in ISA broilers (particularly in females; P<0.05). The muscle lipids of ISA chickens fed the enzyme-supplemented diet contained more C18:1 (particularly males; P<0.05) and total monounsaturated FA (P≤0.01). Ross chickens responded to the formulation based on DAA with an increase in the CP and dry matter content in the breast muscle (P≤0.01). The tendency towards reduced fat levels in the Ross birds fed the DAA diet was significant for females (interaction; P<0.05). In contrast to ISA, in Ross birds added enzymes did not significantly affect the proportions of the analysed muscle components.