Ro09-0198, a cyclic peptide isolated from culture filtrates of Streptoverticillium griseove-rticillatum, induced platelet aggregation and serotonin release simultaneously. LDH release was not observed. Addition of peptide to rat platelet, loaded with $Ca^{2+}$ chelator quin-2, caused immediate rise in cytosolic free $Ca^{2+}$. Liposomal membrane containing phosphatidylethanolamine was damaged by peptide and released $^{45}Ca$ dose dependently.