It has been recently reported that the major inorganic particle components of cigarette smoke are calcium and potassium carbonates, potassium sulfate and chloride, and quartz. To determine whether any of these particles might serve as a marker of cigarette smoke exposure in human lungs, we used analytical electron microscopy to compare the mineral particle content in two parenchymal sites and two large bronchi from five nonsmoker autopsy lungs and five age- and sex-matched smoker autopsy lungs. Potassium carbonate, sulfate, and chloride were not identified in any lung. The percentage of quartz was the same in both smoker and nonsmoker lungs. However, lungs from smokers contained a large percentage (average 23% of all particles) of particles composed of calcium, carbon, and oxygen (probably calcium carbonate) in all sample sites, whereas lungs from nonsmokers usually contained no such particles or only minute numbers (average 0.1%). We conclude that calcium-containing particles of this type may serve as a marker of cigarette smoke exposure in humans and may prove useful for examining the distribution of smoke particles in the normal and diseased lung.