AbstractA radioimmunoassay (RIA) for measurement of platelet‐activating factor (PAF) was developed. At a final antiserum dilution of 1∶640, the lowest detection limit of PAF was 0.1 pmol (50 pg). The standard curve obtained was suitable for measurement of PAF in amounts ranging from 0.1 pmol to 30 pmol. The antiserum showed high specificity. Cross‐reaction for lysoPAF, lysophosphatidylcholine and long‐chain phosphati‐dylcholines was very low (less than 0.025%). 1‐Palmitoyl‐2‐acetyl‐sn‐glycero‐3‐phosphocholine cross‐reacted slightly (6.25%). PAF exogenously added to macrophage suspensions was quantitatively determined by RIA after solvent extraction and high‐performance liquid chromatographic separation. RIA was also used to estimate PAF formation after stimulation of rabbit alveolar macrophages in suspension with calcium ionophore A23187.