Alveolar and bronchiolar spaces in the lungs of cigarette smokers usually contain numerous macrophages with pigmented cytoplasmic granules. By electron microscopy the pigmentation appears to be due, at least in part, to increased numbers of lysosomes and phagolysosomes. Within these cytoplasmic organelles, structures are found which we designate "smokers' inclusions", since they are observed exclusively in the interstitial and alveolar macrophages of cigarette users. The inclusions have been referred to by other investigators as "needle-shaped" and "fiber-like". Since cross-sections of the structures are never seen in electron micrographs, a fiber or needle shape seems unlikely. On the other hand, thin sectioning techniques impart varying lengths to the inclusions, suggesting that they have a disc, or platelet, configuration. Surgically resected lung tissue from smokers and nonsmokers was digested in hot potassium hydroxide. Digestates contained varying numbers of hexagonal platelike particles which had features consistent with those of the aluminum silicate kaolinite, and energy-dispersive x-ray spectrometry confirmed the presence of these two elements. The origin of aluminum silicate inclusions in pulmonary macrophages has yet to be determined, although preliminary evidence strongly suggests that they are derived from inhaled tobacco smoke. The cytotoxicity of kaolinite in vitro and the possible role of aluminum silicate crystals in the pathogenesis of pulmonary fibrosis are discussed.