Abstract This study aimed to determine the effect of NexPro on growth performance, carcass characteristics, and markers of immune function in finishing cattle. Angus-cross steers (n = 210) weighing 465 ± 48 kg were used for this 89-day study. Steers were blocked by BW into four treatments: SBM; a control diet with 11% soybean meal on DM basis; NP9 (9% NexPro), NP12 (12%), and NP15 (15%). NexPro replaced dry-rolled corn in the diet as NP inclusion increased. NexPro treatments each had 9 pens and SBM had 8 pens (6 steers per pen). On day 0, steers were implanted with Component TE-200 (Elanco Animal Health). The BW of all steers and bunk refusals were measured on d -1, 0, 26, 61, 75, and 89 and were used to calculate pen gain, DMI and feed efficiency for each period. One steer per pen was selected for blood collection and muscle biopsies collected on d 26 and 75. From d 61 to 89 steers received 300 mg/d of ractopamine hydrochloride (RAC; Actogain; Zoetis). Fresh blood samples were used for immune cell population determination using flow cytometry. Data were analyzed using the Mixed procedures of SAS with pen as the experimental unit and fixed effects of treatment and block. Performance data were analyzed using d 0 BW as covariate. During the pre-RAC period (d 0 to 61) steers fed the SBM treatment ate more (P < 0.001) than steers fed the NP diets. Steers on the SBM diet had the greatest ADG, with NP12 and NP15 intermediate, and NP9 lowest (P ≤ 0.06), with no treatment differences in G:F (P = 0.26). However, during RAC DMI improved in all NP treatments, and were not different (P = 0.32) from SBM. During the RAC period, ADG tended to be greater (P = 0.09) in NP15 than all other treatments, and G:F was improved (P = 0.01) in NP15 compared with other treatments. Carcass-adjusted BW and ADG, and HCW were less in NP9 (P ≤ 0.02) but similar between SBM, NP12 and NP15. Carcass-adjusted G:F did not differ by treatment (P = 0.50). On d 26, NP12 and NP15 had greater frequencies of live gamma-delta T cells, and on d 75, NP15 had the greatest frequency of these cells (P ≤ 0.016). All NexPro treatments had greater percentages of CD45RO positive B cells on d 75 (P = 0.03) and increased frequency of live total natural killer cells from d 26 to 75 (P = 0.06). The less than expected DMI and greater rumen undegradable protein in NexPro may have caused NP9 to be RDP deficient. However, the greater inclusions of NexPro supported excellent cattle gains, similar or better than SBM.