Development of systems of medicines with sustained action on the basis of biodegradable polymers is a promising trend in modem pharmacology. Polyhydroxyalkanoates (POA) attract increasing attention due to their biodegradability and high biocompatibility, which make them suitable for development of novel drug dosage forms. We obtained microspheres on the basis of poly(3-hydroxybutyrate) (PHB) loaded with the antitumor drug paclitaxel. Morphology, drug release kinetics and effect on tumor cells in vitro of microspheres were studied. The data on the kinetics of drug release, biocompatibility and biological activity of the biopolymer microspheres in vitro showed that the studied system of prolonged drug release had lower toxicity and higher efficiency compared to the traditional dosage forms of paclitaxel.