Ewing's sarcoma (ES) and peripheral neuroepithelioma (PN) are closely related tumors, and it can be difficult to distinguish them from other small-round-cell tumors (SRCTs). The glycoprotein p30/32MIC2 is highly, but not exclusively, expressed in both ES and PN. Although the monoclonal antibody (Mab) HBA71, which reacts with P30/32MIC2, has been reported to be relatively specific and highly sensitive for both neoplasms, it is not readily available. Yet, Mab O13 is commercially available, and it purportedly displays the same immunostaining characteristics as HBA71. Because O13 has not been studied extensively, we immunostained 21 ES/PNs and 147 other tumors or lesions that might show SRCT-like features with O13. The results were similar to those reported for HBA71. We found O13 to be 100% sensitive for ES/PN; and, no immunostaining was noted on the SRCTs often included in the differential diagnosis of ES/PN (i.e., conventional neuroblastoma, rhabdomyosarcoma, and non-lymphoblastic lymphomas). But, O13 immunoreacted with lymphoblastic lymphomas and some other tumors and normal tissues. Nonetheless, this nonspecific reactivity should not cause diagnostic problems, if an antibody panel containing anti-desmin and anti-leukocyte common antigen is used in conjunction with O13. We conclude that, within the proper diagnostic context, strong immunoreactivity of a SRCT tumor for O13 should be considered good evidence that the tumor is ES/PN.