Tumor-directed immunity was assessed in 29 patients with neuroblastoma (NB). Cell-mediated and humoral immunity were assessed in vitro using the one-stage direct capillary leukocyte migration inhibition test and the indirect, membrane immunofluorescence test, respectively. Leukocytes from 83% of NB patients, 40% of normal children, and 24% of adults were inhibited by NB-derived materials. NB patients' leukocytes reacted as frequently with materials from allogeneic neuroblastomas as with autologous extracts. Sera from 50% of N B patients, 8% of child controls, and 18% of adult controls reacted with allogeneic tissue cultured neuroblastoma cells.Leukocytes from NB patients, control children, and control adults were inhibited at a similar low frequency by extracts of human melanomas and cancers of cervix, colon, and breast. Sera from NB patients and controls reacted infrequently with cultured melanoma cells.