We evaluated the effects of MP supply on growth performance before and after preconditioning and measurements of innate and humoral immune response of beef steers following vaccination. Angus steers ( = 36; BW = 231 ± 21 kg; age = 184 ± 18 d) were weaned on d -6, stratified by BW and age on d 0, and randomly assigned to 1 of 18 drylot pens (2 steers/pen). Treatments were assigned to pens (6 pens/treatment) and consisted of corn silage-based diets formulated to provide 85%, 100%, or 115% of the daily MP requirements of a beef steer gaining 1.1 kg/d from d 0 to 42. Steers were vaccinated against infectious bovine rhinotracheitis virus, bovine viral diarrhea (BVDV) types 1 and 2 viruses, and clostridium on d 14 and 28. Blood samples were collected on d 0, 14, 15, 17, 21, 28, 29, 30, 35, and 42. Body weight did not differ ( ≥ 0.17) among treatments from d 0 to 28. On d 42, 115% MP steers were heaviest, 100% MP steers were intermediate, and 85% MP steers were lightest ( = 0.05; 297, 290, and 278 ± 7 kg, respectively). Overall, ADG and G:F did not differ ( ≥ 0.13) between 100% and 115% MP steers and were least ( < 0.01) for 85% MP steers (1.2, 1.4, and 0.8 ± 0.07 kg/d and 0.23, 0.24, and 0.19 ± 0.008, respectively). Plasma haptoglobin (Hp) concentrations did not differ among treatments ( ≥ 0.46), whereas plasma ceruloplasmin (Cp) concentrations were greatest ( ≤ 0.04) for 85% MP steers, intermediate for 100% MP steers, and least for 115% MP steers on d 30, 35, and 42. Plasma cortisol concentrations were greater ( ≤ 0.03) for 85% vs. 100% and 115% MP steers on d 14 and 28. Liver mRNA expression of Cp and Hp and muscle mRNA expression of m-calpain, mammalian target of rapamycin, and ubiquitin did not differ among treatments ( ≥ 0.17). Serum neutralization titers to BVDV-1b titers were greater ( ≤ 0.02) for 115% vs. 85% and 100% MP steers on d 42 (5.8, 3.0, and 3.7 ± 0.60 log, respectively), whereas mean serum leukotoxin titers were greater for 85% vs. 100% and 115% MP steers (3.1, 2.4, and 2.5 ± 0.21 log, respectively). Preconditioning MP supply did not affect ( ≥ 0.26) ubsequent finishing growth performance and carcass characteristics. Thus, increasing MP supply from 85% to 115% of daily requirement of preconditioning beef steers had variable results on innate and humoral immune response and enhanced growth performance during a 42-d preconditioning period without affecting carcass characteristics at slaughter.