Venom from the tropical ant, Pseudomyrmex triplarinus, has activity against rheumatoid arthritis. Since platelets are involved in inflammatory responses, they were employed to study the effects of venom on prostaglandin-dependent human platelet aggregation and secretion. The assay is very sensitive and uses microliter volumes, which makes it useful as a screen during isolation and characterization of venom components. Whole venom inhibited arachidonic acid- and U46619-induced platelet aggregation with IC50s of 45 and 39 micrograms/ml, respectively. This suggested that venom prevented the action of prostaglandins. Pure venom was fractionated by gel filtration and at least three materials with antiplatelet activity were detected. The smallest component (factor F) was most active and was purified by additional molecular filtration and characterized by UV absorbance, thin-layer chromatography, nuclear magnetic resonance spectra, and activity to platelets. Factor F was identified as adenosine, which is known to stimulate platelet adenylate cyclase and has not been previously reported to be a component of insect venom.