This report covers an interlaboratory study of the in vitro toxicity of particulate matter from four cigarettes using the Ames, Neutral Red Cytotoxicity and Micronucleus assays. The study was an ambitious undertaking in a relatively short period of time, with a large number of variables from assay to assay and from laboratory to laboratory, and a large amount of data. Data from the assay with the greatest historical use, the Ames assay, showed the most concordance among the laboratories, while data from both the Neutral red cytotoxicity and the In Vitro Micronucleus assays exhibited much greater variability.