The purpose of this study was to examine the effects of whey protein supplementation on body composition, muscular strength, muscular endurance, and anaerobic capacity during 10 weeks of resistance training. Thirty-six resistance-trained males (31.0 ± 8.0 years, 179.1 ± 8.0 cm, 84.0 ± 12.9 kg, 17.8 ± 6.6%) followed a 4 days-per-week split body part resistance training program for 10 weeks. Three groups of supplements were randomly assigned, prior to the beginning of the exercise program, in a double-blind manner to all subjects: 48 g per day (g·d−1) carbohydrate placebo (P), 40 g·d−1 of whey protein + 8 g·d−1 of casein (WC), or 40 g·d−1 of whey protein + 3 g·d−1 branched-chain amino acids + 5 g·d−1 L-glutamine (WBG). At 0, 5, and 10 weeks, subjects were tested for fasting blood samples, body mass, body composition using dual-energy x-ray absorptiometry (DEXA), 1 repetition maximum (1RM) bench and leg press, 80% 1RM maximal repetitions to fatigue for bench press and leg press, and 30-second Wingate anaerobic capacity tests. No changes (p > 0.05) were noted in all groups for energy in-take, training volume, blood parameters, and anaerobic capacity. WC experienced the greatest increases in DEXA lean mass (P = 0.0 ± 0.9; WC = 1.9 ± 0.6; WBG =-0.1 ± 0.3 kg, p < 0.05) and DEXA fat-free mass (P = 0.1 ± 1.0; WC = 1.8 ± 0.6; WBG = −0.1 ± 0.2 kg, p < 0.05). Significant increases in 1RM bench press and leg press were observed in all groups after 10 weeks. In this study, the combination of whey and casein protein promoted the greatest increases in fat-free mass after 10 weeks of heavy resistance training. Athletes, coaches, and nutritionists can use these findings to increase fat-free mass and to improve body composition during resistance training.