PAF-acether fraction of Ehrlich ascites tumour cells stimulated with the calcium ionophore A23187 aggregated human platelets. The platelet aggregating ability increased with increasing concentration of ionophore and reached a maximum at 10 microM in 30 min. The phosphatidylcholine fraction from unstimulated cells after alkaline methanolysis followed by acetylation also aggregated human platelets. This semisynthetic preparation had biological and chemical properties identical to authentic PAF-acether. Our results show that Ehrlich ascites tumour cells produce PAF-acether in response to ionophore stimulation.