An in vitro micronucleus (MNvit) assay proficiency study was conducted by member laboratories of the CORESTA In Vitro Toxicity Testing Task Force (a Sub-Group since 2015). Given that different procedures (internal or regulatory) are followed by the participating laboratories, the purpose was not to develop a CORESTA Recommended Method but to evaluate the MNvit induction of a mainstream smoke extract from three cigarettes using a common study protocol. This enabled laboratories to evaluate method proficiency, compare results with those of other laboratories, and obtain an external audit of documentation procedures that might identify potential areas for improvement.