Two hundred fifty-five well-characterized formaldehyde-fixed and paraffin-embedded small round cell tumors mainly from children and young adults including 105 neuroblastomas were immunohistochemically analyzed with the NB84 monoclonal antibody raised to neuroblastoma cells. Most of the undifferentiated neuroblastomas (21 of 22) and all 83 differentiated neuroblastomas reacted with NB84, but none of these tumors were CD99 positive. Compared with synaptophysin, NB84 was more sensitive, although less specific, in the identification of neuroblastoma in formaldehyde-fixed tissue. In addition to neuroblastoma, skeletal and extraskeletal Ewing's sarcoma and medulloblastoma showed NB84 reactivity in approximately 20% of cases, and 50% of desmoplastic small round cell tumors showed positive cells, usually in smaller numbers than the neuroblastomas. The NB84 reactivity was seen slightly more commonly in morphologically defined (rosette-positive) cases of peripheral primitive neuroectodermal tumors than in Ewing's sarcoma. However, the NB84 positivity did not correlate with the expression of other neural markers (neurofilament proteins, CD57, and synaptophysin) in these tumors. All other small round cell tumors including rhabdomyosarcomas, Wilms' tumors, and lymphomas were NB84 negative. In the case of NB84-positive tumors other than neuroblastoma, their specific reactivity for other markers was useful (Ewing's sarcoma CD99 positive, desmoplastic small round cell tumor desmin and keratin positive). The NB84 monoclonal antibody is a useful reagent to separate neuroblastoma from other small round cell tumors. In problem cases it is best used in a panel together with other markers that address the significant differential diagnostic alternatives.