Medulloblastoma is the most common primitive neuroectodermal tumor of the central nervous system, and shares some biochemical and immunological features with neuroblastoma. It could be suggested that N-myc expression in medulloblastoma is correlated with primitiveness or cell differentiation, as in neuroblastoma. N-myc expression in two human medulloblastoma cell lines (ONS-76 and -81) was investigated during drug-induced differentiation by immunocytochemistry and Western blotting. The growth rates of the two medulloblastoma cell lines showed a marked decrease and morphological differentiation occurred after treatment with 1 mM dibutyryl cyclic-adenosine 3',5'-monophosphate (dbt-cAMP). The decrease in N-myc expression with differentiation of cells was confirmed by immunocytochemistry and Western blotting. We believe that decreased N-myc expression is correlated with cell differentiation, and a decrease in the primitiveness of medulloblastoma cells.