The paper will consider the use of short-term bioassays to identify antimutagenic and anticarcinogenic substances and to classify them according to the locus of their protective influence, i.e., intracellular or extracellular, and putative mechanism(s) of action. In the extracellular environment, inhibition of formation or uptake of mutagens and inactivation of promutagenic or mutagenic species are examples of antimutagenic mechanisms. Intracellularly, antimutagenic substances have been described as 'scavengers' of radicals, 'blocking agents' (involving at least 3 different mechanisms), and 'suppressing agents'. Additional intracellular mechanisms include alterations in DNA repair processes and/or modification of the genotoxic response to the mutagen/carcinogen. The presentation format will be based on the genetic activity profile (GAP) methodology developed by the authors. The GAPs of known mutagens/carcinogens will be presented together with newly designed profiles for antimutagens/anticarcinogens according to the classification scheme outlined above.