Glioblastoma multiforme (GBM) is a grade IV astrocytoma and the most common malignant brain tumor. Current therapies provide a median survival of 12–15 months after diagnosis, due to the high recurrence rate. The failure of current therapies may be due to the presence, within the tumor, of cells characterized by enhanced self-renewal capacity, multilineage differentiation potential and elevated invasive behavior, called glioma stem cells (GSCs). To evaluate the pharmacological efficacy of selected drugs on six GSC lines, we set up a multiple drug responsivity assay based on the combined evaluation of cytomorphological and functional parameters, including the analysis of polymorphic nuclei, mitotic index and cell viability. In order to understand the real pharmacological efficacy of the tested drugs, we assigned a specific drug responsivity score to each GSC line, integrating the data produced by multiple assays. In this work we explored the antineoplastic effects of paclitaxel (PTX), an inhibitor of microtubule depolymerization, utilized as standard treatment in several cancers, and of valproic acid (VPA), an inhibitor of histone deacetylases (HDACs) with multiple anticancer properties. We classified the six GSC lines as responsive or resistant to these drugs, on the basis of their responsivity scores. This method can also be useful to identify the best way to combine two or more drugs. In particular, we utilized the pro-differentiating effect of VPA to improve the PTX effectiveness and we observed a significant reduction of cell viability compared to single treatments.