Phorbolester-triggered differentiation of SH-SY5Y neuroblastoma cells requires serum and a prolonged activation of protein kinase C (PKC). Under serum-free conditions development of a mature phenotype requires phorbolester in combination with a member of either the insulin-like growth factor (IGF) or the platelet-derived growth factor family. Here we report that basic and acidic fibroblast growth factor (FGF) and epidermal growth factor, but not nerve growth factor, synergistically potentiate phorbolester-induced differentiation. Alone these factors induced a mitogenic response which varied in magnitude, with basic FGF and IGF-I being the two most potent mitogens. However, a combination of basic FGF and IGF-I induced differentiation as judged by morphology and the increase in growth associated protein (GAP-43) and neuropeptide tyrosine mRNA levels. In contrast to the phenotype obtained in the presence of phorbolester, bFGF and IGF-I-treated SH-SY5Y cells retained their capacity to proliferate. Finally, in these cells, the phosphorylation of the endogenous PKC substrate, myristoylated alanine-rich C-kinase substrate (MARCKS), was slightly increased during several days, suggesting an involvement of PKC in the bFGF and IGF-I-induced differentiation.