Indirect immunofluorescence studies were compared with conventional smear cytology in 82 paired bone marrow samples from children with neuroblastoma using monoclonal antibodies (MAbs) BW 575 (neuroblastoma-associated 95 kD glycoprotein) and BW 625 (ganglioside GD2) and tetanus toxin labeling. Congruent results were obtained in 70 of 82, or 85% (positive/positive; negative/negative). In 12 of 82 (15%) patients, bone marrow infiltration was demonstrated by immunofluorescence but not by conventional cytology. As few as 0.01% neuroblastoma cells were reliably detected--in some cases even fewer. Because of antigen heterogeneity, false negative results were obtained in five cases with MAb BW 625, in two cases with MAb BW 575, and in no case with tetanus toxin. No antibodies showed any cross-reactivity to hematopoietic cells in either bone marrow of infants or during regeneration after chemotherapy. We conclude that this panel of antibodies is highly sensitive and specific to detect minimal disease in bone marrow of neuroblastoma patients, which has major implications for the staging procedure, monitoring treatment, early detection of relapse, and assessment of bone marrow status before autologous bone marrow transplantation.