Animal experiments to test for the possible carcinogenic activity of chemicals provide the best and only deeply researched method for the detection or environmental carcinogens for man. The fact that most known human carcinogens give tumors in animals encourages the belief that these tests have validity. However, there are significant differences in the numbers of the exposed populations of men and animals, in the part of the lifespan during which each is exposed, in the metabolic activation of the carcinogens, and in the longevity of men and experimental rodents. For regulatory purposes, we must assume that the results of bioassay in rodents will closely parallel tumor induction in man, although we cannot be sure of this. In some cases, such as rodent bladder tumors associated with bladder stone, or subcutaneous sarcomas arising locally to massive injection of food dyes, there may be reason to reject an association. It is only by continued research into the way in which both man and laboratory animals react to carcinogens that we may hope to refine our methodologies and obtain an accurate, well-defined net to trap potential environmental carcinogens without depriving the community of chemicals, through false associations or false positive results that may be of great value, sociiologically or economically.