Ewing family tumors (EFTs) and prostate carcinomas are characterized by rearrangement of ETS genes, most commonly FLI1 (EFTs) and ERG (prostate carcinomas). Previously, we characterized an antibody against ERG (EPR3864) for detecting ERG-rearranged prostate carcinoma. Because EPR3864 also cross-reacts with FLI1, we evaluated the usefulness of EPR3864 for discriminating EFTs from other small round blue cell tumors (SRBCTs) with immunohistochemistry. Of 57 evaluable EFTs, 47 (82%) demonstrated at least moderate, diffuse, nuclear ERG/FLI1 staining (including 89% and 100% of cases with confirmed EWSR1:FLI1 and EWSR1:ERG fusions, respectively), of which 1, 3, and 43 showed negative, cytoplasmic, or membranous CD99 staining, respectively. Among other SRBCTs (61 cases, 7 types), at least moderate, diffuse, nuclear EPR3864 staining was seen in all precursor B-lymphoblastic lymphomas/leukemias and subsets of Burkitt lymphomas (10%) and synovial sarcomas (45%). In summary, EPR3864 may be useful in detecting EWSR1:FLI1 and EWSR1:ERG rearranged EFTs in addition to prostate carcinomas.