The HBA71 antigen is an M(r) 30,000/32,000 cell surface glycoprotein (p30/32MIC2), encoded by the pseudoautosomal MIC2 gene on chromosomes X and Y, that is expressed in Ewing's sarcomas. Immunohistochemical studies demonstrate a striking specificity for HBA71 among neoplasms of diverse histologic types. In the present study, 43 cases of Ewing's sarcoma of bone were tested for HBA71 expression and six additional immunohistochemical markers regularly used in the differential diagnosis of small round-cell tumors of childhood and adolescence (neuron-specific enolase, vimentin, leukocyte common antigen, cytokeratins, muscle-specific actin, desmin). The study design included (a) random selection of Ewing's sarcoma cases from the files of Memorial Hospital beginning in 1968, (b) blind review of the original histopathologic diagnoses of ES, (c) side-by-side immunohistochemical study of recut histologic specimens, and (d) statistical analysis of immunohistochemical findings in view of clinical outcome. Of the seven antigens studied, only HBA71, neuron-specific enolase and vimentin were expressed in a significant proportion of cases. Forty-one of the 43 cases were HBA71+ (95% sensitivity); of these, 21 were neuron-specific enolase+, 29 were vimentin+, and 15 were both neuron-specific enolase+ and vimentin+. One tumor lacked all antigens, and one was vimentin+ only. Comparison of tumor tissues in five patients obtained before and after cytostatic chemotherapy showed no change in HBA71 expression or in the other antigens tested. Product-limit survival analysis (median disease-free survival was 27.3 months for the study cohort) revealed no significance of neuron-specific enolase or vimentin marker status. These results raise doubts about the usefulness of neuron-specific enolase and vimentin immunohistochemistry to distinguish Ewing's sarcoma from other small round-cell tumors of childhood and adolescence or as prognostic indicators in Ewing's sarcoma. The positive identification of Ewing's sarcoma of bone now becomes a reality using HBA71 immunohistochemistry, either as a sole method or in combination with chromosomal breakpoint analysis. This may result in achieving uniform diagnostic criteria for evaluating the biologic, therapeutic, and prognostic aspects of Ewing's sarcoma and related neoplasms.