An international collaborative study of the mutagenicity of complex environmental mixtures in the Ames Salmonella typhimurium mutation assay was performed. The objectives of the study were: (1) to estimate the inter and intra-laboratory variability associated with the extraction of mixtures for bioassay, (2) to estimate the inter and intra-laboratory variability associated with the Salmonella typhimurium bioassay when applied to complex mixtures, (3) to determine if mutagenicity values can be determined from this collaborative study for issuance with standard reference complex mixtures. The complex mixtures used in the study were selected from standard reference materials (SRMs) which had previously been issued by the U.S. NIST as SRMs 1597 (coal tar), SRMs 1649 (diesel particulate matter) and SRMs 1650 (urban air particulate matter) with certified values for polycyclic aromatic hydrocarbons.