The concentration in human plasma of putative neutrophil-''priming'' cytokines like endogenous pyrogens is known to increase significantly in response to moderate exercise (11). This is characteristic of an acute-phase response. The ability of blood neutrophils isolated from both trained and untrained human subjects (n = 11, 9) to produce microbicidal reactive oxygen species was determined using luminol-enhanced chemiluminescence both before and after one hour of aerobic exercise at 60% VO2max. Irrespective of training and stimulus concentration, exercise nearly always caused significant ''priming'' of the capacity of neutrophils to produce H2O2 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. Although neutrophil oxygenation activity is only one parameter that contributes to immunological status, regular episodes of moderate exercise may increase resistance to infection by priming the ''killing capacity'' of neutrophils. In contrast, prolonged periods of intensive training may lead to increased susceptibility to common infections by diminishing this activity.