For about 25 years various investigators at IIT Research Institute (IITRI) have been developing animal model systems to measure the holistic response to drugs, chemicals, and air pollutants. In 1958, the first report on the effects of ozone (03) on bacterial pneumonia was published by Miller and Ehrlich. That research, supported by the United States Air Force, was the basis for our subsequent programs in which several infectious disease models were used to evaluate the effects of a variety of toxicants. Table 1 shows thirteen infectious disease models and one tumor challenge model we have used in mice, hamsters, guinea pigs, or squirrel monkeys. With the exception of the diethylstilbestrol (DES), all toxicants were administered by the inhalation route. Other investigators are working with a variety of tumor and infectious disease models to evaluate toxicants administered by various routes and Dean et al. (1982b) have summarized this research. As a consequence there is a wide selection of host resistance models available for use.