The distribution of microtubules and platelet-specific glycoproteins (GPIIb/IIIa) in particles was probed by an immunofluorescence method using anti-tubulin and anti-GPIIb/IIIa antibodies to identify whether particles released from a human megakaryoblastic cell line (MEG-01) are platelets. The fluorescence image showing anti-tubulin staining of the particles revealed a characteristic ring structure observed in platelets. Anti-platelet GPIIb/IIIa antibody staining showed an image in which small patches or spots were seen throughout the particle with brighter staining at the periphery. No significant difference was observed between these particles and human blood platelets under immunofluorescent staining. These results show that MEG-01 cells released platelet-like particles.