Neuroblastoma, along with rhabdomyosarcoma, Ewing's sarcoma, and acute lymphoblastic leukemia/lymphoma, is one of the small, round-cell tumors of childhood. All of these malignancies show a propensity to metastasize to bone marrow. Occasionally when the clinical picture is unclear and the tumor is particularly anaplastic, it can be difficult to arrive at a diagnosis by conventional histological and biochemical procedures. In the present study, a panel of nine monoclonal antibodies was used to undertake a detailed analysis of seven bone marrows contaminated with tumor cells: six cases of stage IV neuroblastoma, and one case of stage IV-S neuroblastoma. The antibody profiles obtained were compared with those deduced from the studies of over 20 marrows from patients with acute lymphoblastic leukemia. A comparison of these data with those obtained from the studies of rhabdomyosarcoma and Ewing's sarcoma cell lines and tissues suggests that when high levels of tumor cells are present in the marrow, it is possible to obtain a confident diagnosis of either neuroblastoma or acute lymphoblastic leukemia. In addition, the immunocytological identification of neuroblasts in bone marrow enables accurate staging without histological examination.