The concentration in human plasma of putative neutrophil-“priming” cytokines like endogenous pyrogens is known to increase significantly in response to mod-crate exercise (11). This is characteristic of an acute-phase response. The ability of blood neutrophils isolated from both trained and untrained human subjects (n= ll, 9) to produce microbicidal reactive oxygen species was determined using luminol-enhanced chemiluminescence both before and after one hour of aerobic exercise at 60% V̇O 2 max. Irrespective of training and stimulus concentration, exercise nearly always caused significant “priming” of the capacity of neutrophils to produce H 2 O 2 and HOCl upon stimulation with opsonized zymosan (P< 0.01); however, compared to their untrained counterparts, the activity of cells isolated from trained individuals was depressed about 50% at unit stimulus concentration, both before and after exercise (P< 0.075), whilst remaining unaltered at saturating concentrations.