To elucidate more precisely the biological characteristics of neuroblastomas, we examined four human neuroblastomas heterotransplanted into athymic nude mice NB-39 (undifferentiated type), NB-45 (poorly differentiated type with undifferentiated component), NB-52 (poorly differentiated type), and NB-726 (differentiating type) by electron microscopy, immunohistochemistry, and radioimmunoassay for the peptides in tumors. Ultrastructurally, NB-45, NB-52, and NB-726 contained more numerous and variously sized neurosecretory granules than did NB-39. Immunohistochemistry revealed neurofilament proteins, tyrosine hydroxylase, neuropeptide Y (NPY), and chromogranin A-positive cells in the four tumors in the following order of frequency: NB-726, NB-45, NB-52, and NB-39. NB-726, NB-45, and NB-52, but not NB-39, contained galanin-positive tumor cells. NB-45 and NB-726 harbored a few positive cells for calcitonin gene-related peptide. Furthermore, NB-726 exhibited positivity to leu-enkephalin, met-enkephalin, vasoactive intestinal peptide (VIP), and serotonin. Radioimmunoassay substantiated the results of immunohistochemistry, showing NPY in all tumors and either galanin or VIP in three tumors, excepting NB-39. Average doubling time of the tumor was as follows: 2 days in NB-39, 10 days in NB-45, 22 days in NB-52, and 45 days in NB-726. These results indicate that human neuroblastoma cells have different biological characteristics and reduced growth rate with differentiation in terms of ultrastructure and of peptide production abilities.