Ultrastructure of the bronchopulmonary lavage cells, viz., pulmonary alveolar macrophages (AM), neutrophils, plasma cells, ciliated epithelial cells and lymphocytes from calves 12 h following 3-methylindole (3-MI) ingestion is documented in this study. The AM had increased in number and size, and appeared stimulated as indicated by an elevated number of phagosomes and/or various combinations of structures resembling phagolysosomes. Pseudopodia on majority of the neutrophils were either sparse or absent. Morphologic parameters of heightened secretion were present in the occasionally noticed plasma cells. Ciliated epithelial cells which were only sometimes seen contained necrotic mitochondria. Lymphocytes were rarely encountered and were unaltered by the treatment. Ultrastructure of the bronchopulmonary lavage cells from the 3-MI-exposed calves demonstrates cellular response to the respiratory conditions induced by the chemical.