Uncaria tomentosa inner bark extract is a popular plant remedy used in folk medicine to treat tumor and inflammatory processes. In this study, the anti-tumoral effects of its pentacyclic alkaloid mitraphylline were investigated. Furthermore, its growth-inhibitory and cytotoxic effects on glioma GAMG and neuroblastoma SKNBE(2) cell lines were studied using cyclophosphamide and vincristine as controls. A colter counter was used to determine viable cell numbers, followed by application of the tetrazolium compound [3-( 4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)2-( 4-sulfophenyl)-2H-tetrazolium], inner salt, colorimetric method to evaluate cell viability in this cytotoxicity assay. Micromolar concentrations of mitraphylline (from 5 to 40 µM) inhibited the growth of both cell lines. It inhibited the growth of the two cell lines studied in a dose-dependent manner.