We have developed a mouse monoclonal antibody 5C11 (IgG2a) against cell surface antigen of Ewing's sarcoma (ES). 5C11 specifically reacted with ESs but not with other small round cell tumours in childhood, i.e. neuroblastomas, primitive neuroectodermal tumours (PNETs), rhabdomyosarcomas and malignant lymphomas. 5C11 did not react with any other tumours in children except for hepatoblastomas. No reactivity has been identified in normal tissues with the exception of fetal hepatocytes. Immunoelectron microscopically, 5C11 reactive antigen was located on cell membrane of ES cells. Biochemically, 5C11 immunoprecipitated a cell surface protein having molecular weight of 81,000 Da. 5C11 is the first antibody which can clearly distinguish ES from neurogenic tumours, especially from PNETs which were recently reported to have common features to ESs regarding chromosal abnormality and proto-oncogene expression but show evident differentiation into neurogenic direction. The results strongly indicate the usefulness of 5C11 not only for diagnostic purpose when no specific marker is available but also for studying the histogenesis of ES. In addition, no reactivity in normal tissue implies its potential application as a therapeutic reagent when the management of ES patients is still a great problem in clinical field.