Incubation of 0.5 nM platelet-activating factor (PAF) with 123 micrograms/ml C-reactive protein (CRP) for 60 min at room temperature in the presence of 2 mM calcium resulted in total inhibition of the platelet-aggregating capacity of 0.5 nM PAF. There was no inhibition of platelet aggregation if PAF and CRP were added to the platelets simultaneously, without prior incubation of the mixture, or if calcium was not present during the incubation of CRP and PAF. The inhibitory effect of CRP (123 micrograms/ml) was dependent upon the time of incubation with PAF (1 nM). We observed 0, 42, and 85% inhibition of platelet aggregation with incubations of 0, 20, and 120 min, respectively. The inhibitory effect of CRP was also concentration dependent. After incubations of 20 min at room temperature with 0.5 nM PAF there was no inhibition of PAF-induced platelet aggregation using 12.3 micrograms/ml CRP with the inhibition increasing to 73% using 123 micrograms/ml CRP. Pretreatment of the platelets with CRP (123 micrograms/ml) for 60 min before addition of PAF did not affect the induction of platelet aggregation by PAF. These observations indicate that CRP inhibits PAF and strongly suggest that binding of CRP to PAF, presumably to the phosphocholine moiety of PAF, is required for this inhibition.