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It has been proposed that protein supplementation during resistance exercise training enhances muscle hypertrophy. The degree of hypertrophy during training is controlled in part through the activation of satellite cells and myonuclear accretion.This study aimed to determine the efficacy of protein supplementation (and the type of protein) during traditional resistance training on myofiber cross-sectional area, satellite cell content, and myonuclear addition.Healthy young men participated in supervised whole-body progressive resistance training 3 d·wk−1 for 12 wk. Participants were randomized to one of three groups ingesting a daily 22-g macronutrient dose of soy–dairy protein blend (PB, n = 22), whey protein isolate (WP, n = 15), or an isocaloric maltodextrin placebo (MDP, n = 17). Lean mass, vastus lateralis myofiber-type–specific cross-sectional area, satellite cell content, and myonuclear addition were assessed before and after resistance training.PB and the pooled protein treatments (PB + WP = PRO) exhibited a greater whole-body lean mass %change compared with MDP (P = 0.057 for PB) and (P = 0.050 for PRO), respectively. All treatments demonstrated similar leg muscle hypertrophy and vastus lateralis myofiber-type–specific cross-sectional area (P < 0.05). Increases in myosin heavy chain I and II myofiber satellite cell content and myonuclei content were also detected after exercise training (P < 0.05).Protein supplementation during resistance training has a modest effect on whole-body lean mass as compared with exercise training without protein supplementation, and there was no effect on any outcome between protein supplement types (blend vs whey). However, protein supplementation did not enhance resistance exercise–induced increases in myofiber hypertrophy, satellite cell content, or myonuclear addition in young healthy men. We propose that as long as protein intake is adequate during muscle overload, the adaptations in muscle growth and function will not be influenced by protein supplementation.