Sprague-Dawley rats received intratracheal instillation of 50 mg silica dissolved in 1 ml saline. Animals were killed by intraperitoneal injection of nembutal at periods between 0.5 hour and 4 months. Analysis of bronchoalveolar lavage (BAL) cells and histological and ultrastructural studies were performed on the right and left lungs, respectively. A large number of neutrophils was recovered in BAL fluid from 3 hours after silica infusion, and was maintained until 4 months as the silica stimulus remained in the lung tissues. The absolute number of lymphocytes gradually increased in BAL fluid from day 2. The recovery of macrophages was quite low 24 hours after silica instillation. In contrast, massive aggregates of macrophages were forming silica granulomata in the lung tissues. This discrepancy of results between cells obtained from BAL and those in the lung tissues was caused by an impaired capacity of migration of alveolar macrophages by the development of a junctional apparatus, the "subplasmalemmal linear densities (SPLD)". These structures consisting of hemidesmosome-like unpaired and desmosome-like paired ones infrequently appeared in the acute stages of granuloma formation and were quite common in macrophages and epithelioid-like cells in granulomata 2 weeks after instillation and thereafter. Silicotic granulomata grew in the alveolar ducts from the aggregates of macrophages superimposed on a various number of alveolar sacs. Regeneration of type II alveolar epithelial cells and bronchiolar cuboidal cells usually covered the outer border of the granuloma. Endothelial cells of alveolar capillaries regenerated following recurrent injuries, and only occasionally formed a fenestration structure in their cytoplasm.(ABSTRACT TRUNCATED AT 250 WORDS)