Human neuroblastoma cell line (NB-I), obtained from metastatic neck lymph node of 2.3 y-o, boy, is established and maintained in vitro for more than 3 years in our department. The cells have been proliferated in 10% calf serum in EAGLE's medium, in closed stational culture system. The cell line has characteristic activities of rather rapid growth (Tg = 40 or 50h, 1974) catecholamine metabolism and chromosomal abnormality, bearing two marker chromosomes. NB-I cells are uncloned because of co-adhesiveness but biochemically identified as adrenergic line. Morphologically, they are round or tear-drop like in shape with delicate short protoplasmic processes. NB-I cells exhibit remarkable morphological differentiation and maturation under treatment of (But)2cAMP (final concentration, 1mM/ml). A longer time (1-2 weeks) for differentiation, however, is needed in comparison with that of mouse neuroblastoma cell lines. The NB-I cells exhibit hypertrophy of cytoplasma and nuclei in size, elongation of cytoplasmic processes, as well as the tendency of flattness and multipolarity of cells. Electronmicroscopically, the cells that showed scant organella but plentiful polysomes in the specimen, obtained during autopsy, reveal an increase of cell organellae after implantation in vitro: Golgi apparatus, neurotubules and neural dense bodies appeared in cytoplasma. These organellae characteristic of sympathetic neuron are much more increased under treatment of (But)2cAMP in 3 weeks. Core-vesicle like dense bodies are extremely rich in periphery of perikaryon and elongated processes (100 - 150 mu in diameter). Microtubules are also increased, running in bundles in processes. The NB-I cells are considered to differentiate and approach to matured neurons in vitro.