We studied the effects of two substances related to acute inflammation, platelet activating factor (PAF) and C-reactive protein (CRP), on the chemiluminescence reaction of human neutrophils. PAF caused chemiluminescence dosedependently when calcium ion was present. This luminescence was inhibited competitively by CV-3988, a structural analogue of PAF. However, when CRP, an indicator of inflammation, was added, the luminescence reaction was inhibited. The findings suggested that CRP neutralizes the action of PAF in acute inflammation.