To evaluate the feasibility and clinical usefulness of immunocytochemical detection of bone marrow metastases in neuroblastoma, we studied bone marrow samples from patients undergoing intensive therapy, followed in the majority of cases by autologous bone marrow rescue. Two monoclonal antibodies were used in an indirect immunoenzymatic assay to test 384 samples collected from multiple bone marrow sites during 79 staging procedures in 48 patients. Of 578 immunocytochemical tests, 59 (10%) yielded non-evaluable results. Analysis by individual bone marrow sites showed an agreement between cytological and immunocytochemical examinations in 276 of 309 (89%) evaluable tests with 5 A7 and in 179 of 210 (85%) with UJ 13 A. Infiltration by neuroblastoma cells was reported in 9% of samples by cytology, in 6% by immunochemistry with 5 A7 and in 16% with 13 A. Analysis of results by staging demonstrated agreement between cytological examination and immunocytochemical detection with both monoclonal antibodies in 60 of 75 (80%) evaluable stagings. Bone marrow metastasis was detected by cytology in 22% of stagings, by immunochemistry with 5 A7 in 23%, with UJ 13 A in 25%. Detailed analysis of discordant results revealed that they were related partly to bone marrow sampling variability associated with focal and minimal metastasis of neuroblastoma cells. These data suggest the clinical usefulness of immunocytochemical detection as a complementary test to cytological examination for accurate evaluation of bone marrow infiltration in patients with disseminated neuroblastoma.