High-intensity exercise results in immune activation. This study determined whether (a) there is concordance between serum MPO and neutrophil and/or monocyte intracellular MPO content; (b) peripheral blood mononuclear cells respond to inflammatory interleukins (ILs) by increasing intracellular signaling. Healthy male (n = 12) volunteers participated in high-intensity running (12 × 5 min, 10% decline, 15 km/h). Blood sample (pre, post, 4 h) analyses included serum concentrations of IL-1β, IL-1ra, IL-4, IL-6, IL-8, IL-10, matrix metalloprotease-9 (MMP-9) and creatine kinase (CK). Intracellular IL-6, IL-10, MPO and STAT3/SOCS3 signaling were assessed in mononuclear cells. CK (1573 ± 756 u/L), MMP-9 (101 ± 27 ng/mL), neutrophil (9.89 ± 0.76 × 109 cells/L) and monocyte counts (1 ± 0.08 × 109 cells/L) increased at 4 h. At 4 h serum (7.1 ± 1.3 ng/mL) and monocyte MPO (1.7-fold) increased, whereas neutrophil MPO decreased (0.8-fold). Intracellular monocyte IL-10 and IL-6 decreased by 15% and 20–30%, respectively, coinciding with elevations in serum IL-10 of 14.5 ± 4.7 pg/mL and IL-6 of 5.4 ± 2.9 pg/mL, suggesting immune cell cytokine release in response to exercise. Intracellular PBMC p-STAT3 to total STAT3 ratio increased from pre to 4 h. Circulating monocytes are responsive to increased serum IL-6 suggesting a negative feedback loop via STAT3 signaling.