Bovine neutrophils alter their inflammatory response upon amino acid supplementation, yet research evaluating effects of specific amino acids is scarce. We examined effects of lysine (Lys) and leucine (Leu) on inflammatory responses of neutrophils (PMN) and mononuclear cells (PBMC) from growing Holstein steers (173 ± 3.7 kg BW). Seven steers were used in a 6 × 6 Latin square design with a factorial arrangement of 2 levels of Lys (0 and 6 g/d) and 3 levels of Leu (0, 15, and 30 g/d). Steers were fed a basal diet and infused with all required nutrients except Lys. Treatments were delivered by abomasal infusion. Blood was collected on d 7 of each 7-d period. A hematology analyzer was used to measure blood cell composition, and immune cells were isolated using density gradients. To harvest media for cytokine measures, isolated cells were treated with lipopolysaccharide (LPS, 0 or 1 µg/mL) for 2 (PMN) or 24 h (PBMC); PBMC were treated with concanavalin-A (10 µg/mL) to enhance proliferation. PMN were exposed to dihydrorhodamine and labeled E. coli for 50 min to assess oxidative burst and phagocytosis. Period and steer were considered random effects, and data were transformed (square root or log10) to attain normality. Lys reduced lymphocyte numbers (6.40 vs. 6.61 ± 0.52 × 10³/µL, P = 0.04). Leu (30 g/d) almost doubled eosinophil numbers but only for non-Lys fed steers (0.36, 0.31, and 0.63 ± 0.12 × 10³/µL, P = 0.02). PMN phagocytic and oxidative burst responses were not impacted by treatment. In the presence of LPS, TNF-α produced by PMN (1.7, 8.5, 17.1 ± 6.9 pg/mL, P = 0.08) and IFN-γ produced by PBMC (384, 431, 523 ± 87 pg/mL, P = 0.01) increased or tended to increase linearly as Leu increased. In the absence of LPS, TNF-α produced by PBMC tended to increase quadratically as Leu increased (424, 630, 431 ± 99 pg/mL, P = 0.08). Regardless of LPS, Lys tended to increase production of IFN-γ by PBMC (353 vs. 400 ± 83 pg/mL, P = 0.10). These findings point to a role for dietary Lys and Leu in regulating immune responses.