Adult male cynomolgus-monkeys and Fischer-344-rats of both sexes were used in a chronic inhalation toxicity study designed to evaluate the nature of the lung response to inhalation of fibrous glass, with respect to the prevailing federal standard. The monkeys were exposed to fibrous glass for 7 weeks at the rate of 7 hours per day, 5 days per week. Rats underwent the same exposure for 72 weeks. The overall response was qualitatively similar in both animal species tested. Gross and microscopic pathology generally revealed macrophage responses to inhaled fibers, granulomas containing fibrous glass and translocation of fibers from the lung to draining lymph nodes, with no evidence of distant translocation or carcinogenic changes induced by the fibrous glass. Visible pleural plaques were evident in rats but not in monkeys. Both groups of animals presented a statistically significant increase in the incidence of mononuclear cell leukemia. There was no evidence of a fibrogenic response from exposure to fibrous glass. In rats, the most severe response was recorded following inhalation of glass fibers less than 10 microns long with a diameter of 1 micron, with no binder; the response to fibers having a length greater than 20 microns and a diameter of 4 to 5 microns, with no binder, was minimal. In monkeys, the response was about the same for all sizes of fibrous glass inhaled, except for fibers having a length greater than 20 microns and a diameter of 4 to 6 microns, with binder, which induced minimal response.