To determine if any given chemical is carcinogenic for man is a formidable problem. Only a mere handful have thus far been uncovered, and the total number of cancers causally related to known carcinogens represents only a minute fraction of all cancers affecting man (to detect a mutagenic or teratogenic effect in man is even more difficult). One can suggest that 80 or 90 per cent of cancers are of 'environ° mental' origin but the fact is that the etiology of most cancers is unknown at the present time. Cancer was a major cause of morbidity and death long before food additives and chlorinated hydrocarbon insecticides were an intimate part of our daily life. The fact is that most carcinogens have been detected in the industrial environment, regardless of the availability of the data on animal experiments indicating carcinogenic activity. The information gained in uncovering industrial cancers could perhaps be used more profitably in determining risk of exposure to putative carcinogens. We know that in industrial cancers: (l) the population at risk is small; (2) the exposure is high and concentrated; (3) the incidence of cancer is high; and (4) the latent period is often short, at least in some of the affected individuals (Sternberg, 1976). However, in circumstances where there is a high and concentrated exposure, such as in a manufacturing plant, and where groups of workers have been followed for a reasonable number of years and no increased cancer incidence has developed, it is unlikely that a carcinogenic effect will ever be demonstrated in man. If there is exposure to the same agent in the general population but at a much lower dose and the incidence is such that only one person in, say, 100,000 develops a cancer from the chemical, the causal relationship becomes virtually impossible to detect. How do we assess the danger to man under the circumstances described? One approach (which has many advocates) is to ban everything which has been shown to have a tumorigenic effect in animals. While this may seem to be the safest approach, it is not necessarily the soundest. The decision should be based on benefit versus risk. A food dye generally represents a frivolous additive and can usually be eliminated without affecting our health. On the other hand, the potential danger of nitrites must be evaluated in terms of cancer risk versus the risk of botulism. And, perhaps more importantly, the risk of an insecticide such as DDT as a possible carcinogen vis-a-vis its ability to diminish or eliminate malaria, a disease no less malignant than cancer. Finally, what is considered a valid animal test for carcinogenesis is another thorny problem. Aflatoxin causes liver cancer in mice, rats, dogs, birds and other animal species. It does so with relative ease and with an extraordinarily high incidence in some tests. Such experiments leave little doubt as to the carcinogenicity of aflatoxin