An in vitro micronucleus assay proficiency study was conducted by member laboratories of the CORESTA In Vitro Toxicity Testing Sub-Group. The purpose of the study was not to develop a CORESTA Recommended Method, but to evaluate the micronucleus induction of a mainstream smoke extract of three cigarettes using a common study protocol. The median variability of three replicates was below 20 % and the proficiency study showed that the sensitivity of the micronucleus assay was good enough to differentiate the test pieces. The study enabled laboratories to evaluate the method proficiency, to compare results with those of other laboratories and to obtain an external audit of documentation procedures that might identify potential areas for improvement.