Sphingosine 1-phosphate (S1P) is a bioactive phospholipid that is released by platelets and endothelial cells and has been implicated in diverse biological functions. We hypothesized that S1P may influence immune complex-mediated polymorphonuclear neutrophil activation. Using flow cytometry and fluorescence spectrometry, we found that exogenous addition of S1P led to an enhanced polymorphonuclear neutrophil Fcgamma receptor-mediated rise in intracellular Ca(2+) and reactive oxygen species generation in a pertussis toxin-independent manner, while having only a small effect by itself. Thus, S1P amplifies a positive feedback loop where Fcgamma receptor-mediated rises in Ca(2+) and reactive oxygen species are interdependent, with reactive oxygen species acting to increase tyrosine phosphorylation and activity of upstream signaling intermediates. S1P augmentation of Fcgamma receptor signaling translates to downstream functional consequences, including shape change and recruitment to endothelial surfaces coated with suboptimal levels of immune complexes. Taken together, S1P from activated platelets or endothelial cells may serve to amplify leukocyte recruitment and tissue injury at sites of immune complex deposition in vasculitis.