Some species and strains of experimental animals have such unique mechanisms of developing cancer that the extrapolation of such bioassay results to the human situation would be fraudulent. This fraudulent extrapolation could occur both qualitatively and quantitatively. Although it will be expensive, species other than the rat, mouse, and hamster should be tested, and tested at wider dose ranges than presently used, before risk assessors will have sufficient data to make legitimate risk estimates. Both species- and strain-unique mechanisms and pharmacokinetic information must be made available to the risk assessors before their estimates can be any better than "guesstimates." As more and more data become available, it will become essential that newer techniques of visualization of the data be used in order to evaluate the weight of evidence that an animal carcinogen is or is not a human carcinogen.