The authors observed that Qi-training enhances immune function and modulates neurohormone concentrations. The exact signal and priming mechanism for enhanced neutrophil function by Qi-training has not yet been demonstrated. This study investigated the effect of Qi-training on intracellular signaling leading to the enhancement of immune function. The growth hormone (GH) concentrations and O2- production by neutrophils (PMNs) was significantly increased after 1 h of Qi-training compared with the basal state. To verify that endogenous GH mediates the priming of PMNs, serum obtained from elderly subjects in the basal state and after Qi-training was incubated with neutrophils isolated from young subjects for 60 min and triggered with N-formyl-1-methionyl-1-leucyl-1-phenylalanine (fMLP). Significant O2- production was observed in the PMNs incubated with serum collected after a Qi-training (p < .05). On the other hand, the priming effect on the PMNs was abolished in Qi-training sera depleted of endogenous GH with anti-human GH polyclonal antibody (p < .01) and the tyrosine kinase inhibitor, genistein (p < .01). The authors suggest that the endogenous GH released during and immediately after Qi-training mediates the priming events through tyrosine kinase activation in PMNs.