The protozoan Tetrahymena pyriformis contains 4.2 ± 2.2 ng PAF/107 cells. Only 1–3% of this lipid is released in the cell free medium. PAF production is not influenced by different extracellular Ca2+ concentrations. Cell stimulation with calcium ionophore A23187 or zymosan particles does not affect the amount of PAF either. This is the first report of a natural occurrence of PAF in a protozoan.