Immunization of a BALB/c mouse with cells from the human neuroblastoma line LAN-1 and fusion of the spleen cells with mouse myeloma cells, NS-1, led to the production of a monoclonal antibody (Mab) with a rather unique reactivity for neuroblastoma. This Mab, named 5 A7, detects an antigen of an apparent molecular weight of 65,000-67,000, localized mainly in the cytoplasm and released into culture medium, as revealed by immunoprecipitation and immunoblotting experiments. By immunoperoxidase staining using a biotin-avidin technique, Mab 5 A7 demonstrates a restrictive staining for neuroblastoma cell lines. Following extensive testing on freshly frozen specimens of neuroblastoma and other tumors, Mab 5 A7 shows a highly selective reactivity for neuroblastomas (13 of 14) and some cells of one primitive neuroectodermal tumor (ependymoblastoma). No reactivity could be detected with Mab 5 A7 on the 57 other tumor tissues tested. Among the normal fetal or adult tissue specimens tested, positive staining is found only on adult brain, colonic crypts, some renal tubules, and fetal medulla of the adrenal gland. Among bone marrow specimens tested, only those infiltrated by neuroblastoma cells gave a positive staining. Normal or malignant hematopoietic cells showed no reactivity with Mab 5 A7. Our results with Mab 5 A7 suggest that this reagent not only provides a valuable probe for the immunohistological diagnosis of neuroblastoma on fresh tumor specimens but also allows the detection of bone marrow infiltration by neuroblastoma cells.