Rats, hamsters, and guinea pigs were exposed to airborne glass fiber at a gravimetric concentration of 0.42 mg. per liter for 6 hours per day, 5 days per week for 90 days. The number of dust particles greater than 5 micron. in length was 0.73 x 10(6) per liter with an average diameter of approximately 1.2 micron. Most particles were less than 2 micron. in size and only 15% of the dust particles had a fibrous shape. Few fibers were longer than 10 micron. The pulmonary response was characterized by macrophage reaction with alveolar proteinosis at 90 days of inhalation. The light and ultrastructural alterations were similar to the other experimental or human alveolar proteinosis. The alveolar proteinosis disappeared at 1-year postexposure, but focal dust cell accumulation with proliferating granular pneumocytes persisted throughout the 2-year recovery period. No significant fibrosis or stromal changes were found in the dust-deposited areas. In hamsters and guinea pigs, most ferruginous bodies were developed from fibrous fibers but not from tiny dust particles. The tracheobronchial lymph nodes were markedly swollen and laden with dust cells.