Soluble CD40L primes the PMN oxidase. PMN priming assays were completed with incubation of PMNs with buffer, sCD40L (1 ng/mL-1 μg/mL), or PAF (2 μM) (positive control). The ability of sCD40L to prime the fMLP-activated oxidase in human PMNs was measured over a range of concentrations for 5 minutes. Significant priming of the PMN oxidase occurred with the addition of 10 ng/mL, 100 ng/mL, and 1 μg/mL sCD40L compared with buffer-treated controls (#P < .05; *P < .05; n = 7); significant differences exist between groups with different symbols. Data are presented as mean ± standard error of the mean.