Background: Phthalic acid esters are widely used to improve the plasticity of PVC in medical devices (MD). The most famous plasticizer is DEHP, whose use in medical devices has been contested by the European authorities since 2008. Several alternative plasticizers are being considered to replace DEHP, such as DEHT, TOTM, DINP or DINCH, but they are also released from the PVC throughout their life cycle and are metabolized in the same way as DEHP. Objectives: Our study focuses on the in vitro cytotoxicity of two alternative plasticizers (DINCH and DINP) contained in certain medical devices. They are likely to migrate and be transformed in vivo into the primary and secondary metabolites by a metabolism similar to that of DEHP. This preliminary study is the first to assess the in vitro cytotoxicity of oxidized metabolites of DINCH and DINP based on the EN ISO 10-993-5 standards documents. Methods: We have studied the complete multi-step organic synthesis of secondary metabolites of DINP and DINCH and have performed cytotoxicity tests on L929 murine cells according to the ISO-EN ISO 10993-5 standard design for the biocompatibility of a MD. The tested concentrations of obtained metabolites (0.01, 0.05 and 0.1 mg/mL) covered the range likely to be found for DEHP (total metabolism) in biological fluids coming into direct contact 2 with the MD. The concentrations tested in our study were chosen based on a complete transformation of the plasticizers released after direct contact between a MD and the patient's blood. Results Only 7-oxo-MMeOCH is cytotoxic at the highest concentration (0.1 mg/mL) after 7 days of exposure, just like 5-oxo-MEHP for the same concentration. By contrast, 7-OH-MMeOP, 7-cx-MMeOP, 7-oxo-MMeOP, 7-OH-MMeOCH and 7-cx-MMeOCH were not found to be cytotoxic. Conclusion: The known concentrations of these secondary metabolites in urinary samples are in the μg/L range, i.e. about 100-1000 times lower than the concentrations tested in this study. Cytotoxicity is known to be dose-dependent but it is not always the case for endocrine perturbations and the secondary metabolites could induce endocrine perturbations at very low doses.