Standard cell culture tests according to ISO 10993 have only limited value for the biocompatibility screening of degradable biomaterials such as Mg alloys. The correlation between in vitro and in vivo results is poor. Standard cytotoxicity tests mimic the clinical situation to only a limited extent, since in vivo proteins and macromolecules in the blood and interstitial liquid will influence the corrosion behaviour and, hence, biocompatibility of Mg alloys to a significant extent. We therefore developed a modified cytotoxicity test simulating the in vivo conditions by use of bovine serum as the extraction vehicle instead of the cell culture medium routinely used in standard cytotoxicity testing according to ISO 10993-5. The modified extraction test was applied to eight experimental Mg alloys. Cytotoxicity was assayed by inhibition of cell metabolic activity (XTT test). When extraction of the alloy samples was performed in serum instead of cell culture medium the metabolic activity was significantly less inhibited for six of the eight alloys. The reduction in apparent cytotoxicity under serum extraction conditions was most pronounced for MgZn1 (109% relative metabolic activity with serum extracts vs. 26% in Dulbecco's modified Eagle's medium (DMEM)), for MgY4 (103% in serum vs. 32% in DMEM) and for MgAl3Zn1 (84% vs. 17%), resulting in a completely different cytotoxicity ranking of the tested materials when serum extraction was used. We suppose that this test system has the potential to enhance the predictability of in vivo corrosion behaviour and biocompatibility of Mg-based materials for biodegradable medical devices.