We evaluated the utility of an immunocytochemical technique employing the commercially available anti-CD56 monoclonal antibody, NKH 1. The utility and sensitivity of this technique in the detection of invasive neuroblastoma (NB) cells in the bone marrow were compared with those of Wright-Giemsa staining. The correlation coefficient for the percent NB cells detected using Wright-Giemsa staining with the percent NKH 1 immunoreactive cells was 0.78. In the analysis of specificity, this monoclonal antibody showed slight cross-reactivity with normal bone marrow cells, including macrophages, lymphocytes and osteoblasts. In the evaluation of the sensitivity of the NKH 1 immunocytochemical technique, SK-N-DZ and SK-N-SH NB cell lines were added to morphologically normal bone marrow mononuclear cells from patients without NB to the final NB cell line at concentrations of 2%, 1% and 0.1%. NB cells at the final concentration of 0.1% could be detected by the immunocytochemical technique. We conclude that the NKH 1 immunocytochemical staining technique is useful in the detection of metastatic NB cells in bone marrow.