In view of the potent bronchoconstrictive and inflammatory potency of platelet activating factor (PAF-acether), experiments were undertaken to study the presence of PAF-acether in free lung cells after exposure to cotton dust. Guinea pigs were exposed to airborne standard cotton dust at a concentration of 25-30 mg/m³ for 40 minutes. A lung lavage was performed at different times after the exposure and macrophages and neutrophils were separated and lysed. The lysate was analyzed for PAF-acether activity using serotonin release from rabbit platelets. An increased activity was found 2 hours after the exposure, which declined to zero within 4 hours after exposure.