Platelet-activating factor (PAF) has been considered to be released in a soluble form from activated platelets. Also a considerable number of microparticles were released from stimulated platelets. To evaluate the possibility that microparticles contain PAF, the amount of PAF in whole activated platelets, microparticles and the suspending medium was determined by means of bioassay. The total amount of PAF released from 3 x 10(8) platelets activated with thrombin (0.5 U/ml) plus collagen (10 micrograms/ml) for 10 minutes was 4.6 +/- 1.5 x 10(-12) mol (n = 8, mean +/- S.D.). Eighty-seven percent of the PAF released from platelets was associated with microparticles. PAF released in microparticles is susceptible to plasma acetylhydrolase and may be localized on the outer surface of microparticles. This is the first report implicating the role of microparticles as a novel carrier of mediators formed in the cells.