Valproic acid (VPA) is a simple branched-chain fatty acid that has anticonvulsant activity and is widely used in the treatment of epilepsy. VPA was found to effect growth and differentiation of human neuroblastoma (NB) cells in vitro at concentrations that have been achieved in humans with no significant adverse effects. Treatment of UKF-NB-2 and UKF-NB-3 NB cell lines with VPA at concentrations ranging from 0.5 to 2 mM resulted in neuronal morphological differentiation characterized by extension of cellular processes without significant effects on cell viability. Ultrastructural features of VPA-treated cells were consistent with the neuronal type of differentiation. VPA treatment of NB cells was associated with decreased expression of N-myc oncoprotein and increased expression of neural cell adhesion molecule in their membrane. Treatment of NB cells with 0.5 mM VPA increased their sensitivity to lymphokine-activated killer lysis. The results indicate that VPA, at non-toxic pharmacological concentrations, arrests the growth, induces differentiation and increases immunogenicity of NB cells through non-toxic mechanisms.