Differentiation of human neuroblastoma (NB) was studied in vitro with five NB cell lines treated with dibutyryl cyclic adenosinemonophosphate and retinoic acid. Although the above agents induced different responses in the various cell lines, three overall morphologic phenotypes emerged: a neuronal, characterized by cell processes and neurosecretory granules, a flat cell without pigment, which displayed basal lamina pertinent to Schwann cells, and a flat pigmented cell which exhibited melanosomes, similarly to melanocytes. The activity of the Schwann cell enzyme cyclic nucleotidyl phosphohydrolase increased considerably in one condition, after induction of a predominantly flat cell phenotype. All studied NB cell lines were capable of synthesizing and expressing the extracellular matrix proteins laminin (LM), fibronectin (FN), and Type IV collagen; but a specific pattern of expression emerged after differentiation, which was proportional to normal tissue equivalents: neuronal--none; melanocytic--FN only; and Schwann cell--large amounts of FN, LM, and Type IV collagen.