Novel paclitaxel-loaded polymer nanoparticles were developed for circumventing multidrug resistance (MDR) of malignant cancerous diseases, which is an unsolved clinical problem in cancer chemotherapy. In many cases, MDR is due to the intrinsic or acquired expression of an efflux pump, the P-170 glycoprotein (P-gp). By encapsulating paclitaxel in a water-soluble and biocompatible synthetic polyampholyte using a solid-state reaction the highly water-soluble paclitaxel-loaded nanoparticles are formed. The resulting paclitaxel nanoparticles with an average diameter of 250 nm show a significant reversal of chemoresistance in the drug-resistant variants (MCF7/ADR, MT3/ADR) by a factor of 100 or more. The novel paclitaxel nanoparticles enter MDR breast cancer cells by adsorptive endocytosis bypassing the P-gp, preventing the efflux of paclitaxel and thus restoring the anti-proliferative effect of paclitaxel.