The capacity of platelet-activating factor (PAF) to stimulate its own synthesis by human monocytes was examined. Adherent human monocytes of greater than 85% purity were incubated with 100 fM to 10 nM of PAF in the presence of 20 microCi of [3H]acetic acid to radiolabel newly synthesized PAF. After incubation for 15 minutes to 15 hours, PAF was purified by high-performance liquid chromatography, and newly synthesized PAF was quantified by its radioactivity. PAF stimulated its own synthesis in a dose-related manner with a maximal twofold to threefold increase in synthesis at 10 pM to 100 pM of PAF. Maximal PAF synthesis occurred after incubation for 6 to 8 hours. There was a good correlation (r = 0.95) between PAF quantified by [3H]acetic acid incorporation and by rabbit platelet aggregation bioassay, indicating that the radioactive material is PAF. The protein synthesis inhibitor, cycloheximide, did not inhibit delayed PAF synthesis, indicating that delayed PAF synthesis does not require protein synthesis. PAF is metabolized rapidly in vivo. The capacity of PAF to stimulate its own synthesis would result in a prolonged effective half-life in vivo. This prolonged half-life could contribute to the capacity of PAF to induce prolonged inflammatory reactions in vivo.