In a collaborative study organized under the JEMS MMS, nine mouse lymphoma assay (MLA) "unique positive' NTP rodent carcinogens were re-evaluated by an in vitro chromosomal aberration assay using Chinese hamster lung fibroblast cells (CHL/IU). Six of nine chemicals induced chromosomal aberrations; bromodichloromethane, chlorendic acid and isophorone induced structural aberrations, and chlorodibromomethane, pentachloroethane and 1,1,1,2-tetrachloroethane induced numerical aberrations (polyploidy). These six chemicals, therefore, are not uniquely positive in the MLA. The difference between the NTP results and ours might be due to the use of different cell lines and protocols, and in some cases, to different interpretations of polyploidy. The remaining three chemicals, benzyl acetate, cinnamyl anthranilate and trichloroethylene, were negative in this study.