Plasma and not PGN is limiting for platelet activation. PGN (10 or 20 μg/mL final concentrations) was incubated with the indicated amount of plasma (final concentrations, v/v) for 30 minutes at 37°C before adding to washed platelets for 15 minutes. Platelets were fixed, and the percent of PAC-1–expressing platelets was determined by flow cytometry. The data are the average from 3 independent experiments.