Platelet activating factor obtained from antigen-stimulated rabbit buffy coat leukocytes containing 10% IgE-sensitized basophils was shown by gas-liquid chromatographic and mass spectrometric analysis of products after acetolysis and by base and acid hydrolysis to be acetyl-alkylglyceryl phosphorylcholine. The alkyl side chains were composed of octadecyl (greater than 90%) and hexadecyl (less than 10%) residues.