Experimental evidence suggests that chemical mutagenesis and carcinogenesis are related phenomena such that identification of causative agents and protection from exposure might prevent certain human cancers and related diseases. Agents that inhibit the processes of mutagenesis and carcinogenesis, particularly naturally occurring agents, might be expected to exert a primary protective effect. The paper considers the use of short-term bioassays to identify antimutagenic and anticarcinogenic substances and to classify them (cf. Mutation Res. 168 (1986) 47-65) 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 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.