The critical function of the neurofibromatosis type 1 (NF1) gene product (neurofibromin) is not well defined except that neurofibromin has homology with a family of the GTPase-activating proteins (GAPs). In this study, we confirmed that neurofibromin is constitutively phosphorylated and detected kinase activities which specifically phosphorylate the cysteine/serine-rich domain and the C-terminal domain of the neurofibromin in cell lysate. In vitro and in-gel kinase assays strongly indicated that cAMP-dependent protein kinase (PKA) is a candidate for the neurofibromin kinase. THe biological significance of the phosphorylation of neurofibromin is unclear at present, but we speculate that neurofibromin plays a crucial role in cellular function since it links the two major cellular pathways which are the GAP-ras and PKA-associated signals.