The morphological peculiarities of immunocom- petent cells that were encountered in broncho- pulmonary lavage fluids of the Leghorn chickens were investigated at the electron microscopic level Four distinctive cell types; namely heterophils, eosinophils, lymphocytes, and macrophages, the latter cells predominated. Luminal heterophils exhibited a wide range of ultrastructure heterogeneity, which were strongly suggestive for both active phagocytic and secretory functions. Luminal eosinophils were recognized mainly through their extremely elongated rod-like electron dense cyto- plasmic granules. The outer contour of the luminal eosinophils was mostly broken by comparatively large pseudopodia, and their cytoplasm were packed with many vacuolar and vesicular structures, The cytoplasm of luminal lymphocytes was characterized by abundant ribosomes, few rER tubules, and electron-dense membrane-bound granules. Such criteria were clearly suggestive for a well-developed secretory nature for luminal lymphocytes. Luminal macrophages represented the predominant cell type. Their ultrastructure features were strongly suggestive for an active phagocytic function within the lumen of the chicken's air passageways.