This report summarizes the methodology and findings of a large international collaborative study designed to establish to most useful short-term in vivo tests for confirming the mammalian genetoxicity and carcinogenic potential of chemicals. In view of the importance of the study, key findings have been summarized and interpreted in an effort to make this information accessible to the largest possible audience. The study, which involved the collaboration of 97 investigators from 16 countries, evaluated some 50 different "in vivo" techniques. The objective of the study was to generate a comprehensive data profile from a broad range of short-term test procedures as a means of understanding how various genetic endpoints in key target tissues respond to chemicals defined as genotoxic in vitro. Noting that short-term assays offer considerable advantages in terms of time, expense, and the sparing of laboratory animals, the study also aimed to dispel some of the controversy and confusion regarding precisely which tests and test batteries are needed to produce a reliable and sensitive assessment of the carcinogenic potential of chemicals in mammals, including humans. The opening section explains the purpose and methodology of the study, including criteria used to define an acceptable short-term "in vivo" test, criteria for the selection of test chemicals, and the source and purity of the chemicals. To facilitate evaluation, the 50 test procedures included in the study are divided into six groups: cytogenetic studies, liver-specific assays, miscellaneous assays, mouse spot tests, mammalian germ cell studies and drosophila assays.