Neuroblastoma, a tumor of the sympathetic nervous system, is the most common solid malignancy of childhood outside the central nervous system. Vasoactive intestinal peptide (VIP) is produced by some of these tumors, and elevated serum levels correlate with tumor cell differentiation and a favorable prognosis. It has previously been demonstrated that human neuroblastoma cell lines LA-N-5 and IMR-32 will differentiate in vitro when exposed to retinoic acid. It is now shown that VIP also induces in vitro differentiation of these neuroblastoma lines. LA-N-5 or IMR-32 cells were grown in the presence of different concentrations of VIP. Cell proliferation was suppressed, as measured by cell count, incorporation of [3H]thymidine, and measurement of the proliferation index. The degree of suppression correlated with the concentration of VIP, and the effect was indistinguishable, on a molar basis, from that seen when cells were treated with retinoic acid. Similarly, the morphological changes seen in the VIP-treated cells were the same as those seen in retinoic acid-treated ones. The effects of VIP on both cell lines, like those of retinoic acid, are reversible. The human neuroepithelioma line CHP-100, is much less sensitive to either agent. Vasoactive intestinal peptide is the first substance shown to cause differentiation of neuroblastoma cells in vitro which is also known clinically to have a specific association with that tumor. It is postulated that VIP may play a key role in the well-documented maturation of these tumors in vivo and in the normal development of the sympathetic nervous system. These findings may also have therapeutic implications for the management of this frustrating childhood malignancy.