The purpose of this study was to evaluate whether platelet-activating factor (PAF) is released in the plasma of patients with acute poststreptococcal glomerulonephritis, because previous studies have shown an involvement of platelets in the active phase of this disease. The results of this study indicate that PAF bioactive material is released in significant amounts in the plasma of 49 out of 50 patients with acute poststreptococcal glomerulonephritis. The observed release of PAF was not a direct consequence of the bacterial infection, because it was minimal or absent in patients with streptococcal infection without glomerular involvement. PAF bioactive material extracted and purified from the plasma of patients was shown to be chemically and biologically identical to the synthetic C-16 PAF (1-O-hexadecyl-2-acetyl-sn-glyceryl 3-phosphorylcholine).