Inflammatory cells and lymphocyte populations were examined in the bronchoalveolar lavage (BAL) fluid, lung tissues, and peripheral blood from rats at various times after the intratracheal instillation of silica. In lavage fluid, there was a rapid initial increase in the percentage and number of polymorphonuclear leukocytes (PMN), which slowly decreased during the course of the experiment. In addition, compared to controls, there was an increased number and percentage of lymphocytes throughout the 75 days of the experiment. The lymphocytic populations, which were determined by an indirect immunofluorescence method with monoclonal antibodies to lymphocyte surface markers, showed a predominance of the T-helper phenotype from Day 14 through the end of the experiment (Day 75). The number of PMNs obtained from collagenase digest of the lung was increased over control levels up to Day 7 after silica administration and remained at a relatively constant level until Day 14, after which time they decreased slightly in number. The total number of lymphocytes peaked on Day 14, with cells of the T-helper phenotype predominating after this time. In the peripheral blood, T-helper cells from silicotic rats were significantly increased over control rats on Days 7 and 14 but returned to normal control values after this time. The lymphocyte subsets in the BAL, but not in the peripheral blood, more closely reflect the lymphocyte patterns in the lung. The results of these experiments suggest that T-helper cells may play an important role in the inflammatory-fibrotic events in the lungs of rats with silicosis.