To improve the bioactivity and degradation behavior of biodegradable magnesium, biodegradable metal matrix composites with the ZK30 magnesium alloy as the matrix and bioactive glass (BG, 45S5) as the reinforcement were prepared. The microstructures of the ZK30-BG composites showed homogeneous dispersion of BG particles throughout the matrix. XRD and EDX analyses confirmed the retention of the morphological characteristics and composition of BG particles in the composites. Immersion tests in the minimum essential medium with Earle's balanced salts at 37°C showed that the composites with 5 and 10% BG had lower rates of degradation and hydrogen evolution than the matrix alloy. In addition, the tests confirmed that the composites possessed an enhanced ability to induce calcium and phosphate ion deposition on sample surfaces during degradation, suggesting accelerated surface mineralization that would lead to improved bioactivity when compared with the matrix alloy. In vitro cytotoxicity tests showed that the ionic products of the composites formed during degradation possessed a superior ability to support the survival, proliferation, and osteoblastic differentiation of bone marrow stromal cells to those of the ZK30 alloy. The ZK30-BG composites with enhanced bioactivity and reduced degradation rate could be promising biodegradable materials for orthopedic implants. © 2011 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 100B: 437–446, 2012.