An Ames Assay proficiency study was conducted by member laboratories of the CORESTA In Vitro Toxicity Testing Sub-Group. 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 mutagenicity of a mainstream smoke extract from two cigarettes using a common Study Plan. This enabled laboratories to evaluate method proficiency, compare results with those of other laboratories, and obtain an external audit of their documentation procedures that might identify potential areas for improvement. Each laboratory therefore was able to benefit both technically (scientific methods) and administratively (documentation procedures) from their participation.