Studies of pleural clearance of calcium tungstate particles were made in the dog. By using scanning electron microscopy and elemental microanalysis, we show that mesothelial cells of the visceral leaflet of the pleura are also involved in the clearance of particles present in the pleural space. The histological study of lu g parenchyma shows many macrophages loaded with tungsten particles, and we conclude that this way may be an important pathway for the transmission of pathologic processes from the pleural space to the lung.