This article is in response to the challenge issued to the scientific community by the National Toxicology Program to predict the carcinogenicity potential of 30 chemicals previously selected for long-term carcinogenicity testing. Utilizing the available toxicologic, genetic, and structural information on 30 chemicals previously selected for long-term carcinogenicity testing, we predict that 16 chemicals (53%) would induce some indication of carcinogenic activity in rodents; we further predict that 10 chemicals (33%) would be associated with weak or equivocal carcinogenic responses, and another 4 (13%) would give no indication of carcinogenicity. Our level of certainty is indicated for many of these predictions. Nonetheless, we believe that most instances of guessing whether a chemical would eventually induce cancer in experimental animals and hence represent a carcinogenic hazard to humans are fraught with considerable uncertainty: uncertainty that can only be relieved by long-term testing for carcinogenicity in animals or by conducting an epidemiologic investigation of exposed individuals or groups. We further believe that the day may come when our predictive acumen will be upgraded to such an extent that we might eventually obviate cancer testing. Until then, and in the best interests of public health, however, we urge long term testing of chemicals in animals be continued, at increased pace.