In an effort to establish some of the parameters which influence differences in interpretation of chromosome aberrations, we initiated a collaborative study with 26 different laboratories located in the U.S., Canada and Europe. They include government, private industry, and academic institutions. Three species were selected for study which vary in chromosome number and complexity of the karyotype. These were the Chinese hamster, human, and African green monkey. Each metaphase was preselected and numbered. This allowed each participant in the study to screen exactly the same cells. Most of the participating laboratories have reported the results of their analyses of the Chinese hamster series, and that data is now being analyzed. A detailed discussion of the study format and the preliminary results on the Chinese hamster cells will be presented.