Comments on extrapolation of cancer response from high dose to low dose.

In making judgments as to the cancer risk from low level exposure to carcinogens it is generally necessary to base these judgments on high dose or high incidence data from laboratory or epidemiological studies. The biological considerations involved in making such extrapolations are discussed, as well as some of the mathematical procedures. The difficulties presented by moving from species to species, or from differently acting carcinogenic agents is considered.