Two experiments were designed to evaluate the effects of different probiotic combinations on rumen fermentation characteristics, performance, and carcass characteristics of feedlot Bos indicus beef bulls offered a high-concentrate diet. In Exp. 1, 30 rumen-fistulated Nellore steers were blocked by initial body weight (BW = 350 ± 35.0 kg) and within blocks (n = 10), animals were randomly assigned to receive: 1) high-concentrate diet without probiotic supplementation (n = 10; CONT), 2) CONT plus 1 g/head of a probiotic mixture containing three strains of E. faecium and one strain of S. cerevisiae (3.5 × 10 9 CFU/g; n = 10; EFSC), and 3) CONT plus 2 g/head of a probiotic mixture containing Bacillus licheniformis and B. subtilis (3.2 × 10 9 CFU/g; n = 10; BLBS). The experimental period lasted 35 d, being 28 d of adaptation and 7 d of sampling. From d 34 to 35 of the experimental period, ruminal fluid and fecal samples were collected every 3-h, starting immediately before feeding (0 h) for rumen fermentation characteristics and apparent nutrient digestibility analysis, respectively. In Exp. 2, 240 Nellore bulls were ranked by initial shrunk BW (372 ± 34.9 kg), assigned to pens (n = 4 bulls/pen), and pens randomly assigned to receive the same treatments as in Exp. 1 (n = 20 pens/treatment). Regardless of treatment, all bulls received the same step-up and finishing diets throughout the experimental period, which lasted 115 d. In both Exp., data were analyzed as orthogonal contrasts to partition specific treatment effects: 1) Probiotic effect: CONT vs. PROB, 2) Probiotic type: EFSC vs. BLBS (SAS Software Inc.). In Exp. 1, no contrast effects were observed on nutrient intake, overall nutrient digestibility, and rumen fermentation analyses (P ≥ 0.13). Nonetheless, supplementation of probiotics, regardless of type (P = 0.59), reduced mean acetate:propionate ratio and rumen ammonia-N concentration vs. CONT (P ≤ 0.05). In Exp. 2, no significant effects were observed for final BW and DMI (P ≥ 0.12), but ADG and FE tended to improve (P ≤ 0.10) when probiotics were offered to the animals. Probiotic supplementation or type of probiotic did not affect carcass traits (P ≥ 0.22). In summary, supplementation of probiotics containing a mixture of E. faecium and S. cerevisiae or a mixture of B. licheniformis and B. subtilis reduced rumen acetate:propionate ratio and rumen ammonia-N levels and tended to improve performance of feedlot cattle offered a high-concentrate diet.