ABSTRACT
The carcinogenic potential of test substances is evaluated so that regulatory agencies can
make risk assessments or risk/benefit analyses for exposure of humans to chemicals or
drugs, regulate exposure conditions, and label products appropriately. Because of small
sample size, difficulties in quantification of exposure, variation in individual susceptibility,
the long time to tumor in humans, and other confounding factors, carcinogenicity is
generally difficult to establish by epidemiology. Therefore animal studies are relied upon
at this time to estimate the risk to humans. The current design of assessing carcinogenicity
of chemicals in lifetime studies in rodents was developed by the Weisburgers (1) in the late
1960s and was further refined by the National Cancer Institute/National Toxicology
Program (NTP) (2-4), U.S. Interagency staff (5).
