in humans because of differences in dose, route, and timing of exposure. Second, for the most part, animals are exposed to a single agent and are generally healthy when exposure occurs. Although most of the people exposed to TCDD who are of interest in this report were healthy, they were certainly not exposed solely to TCDD. Third, the toxicity of a given compound varies widely depending on the health status (as determined by nutrition, age, infection, etc.) of the animal examined. When data are available, the contribution of nutrition, age, and other possible factors to the toxicity of the compounds is discussed. Fourth, there is a wide variability in the toxicity of TCDD depending on the species of animal tested. These differences are exemplified in the dose of TCDD required to kill 50 percent of the animals exposed (LD50) (Table 4-1).