The histological characteristics of pleural plaques in workers exposed to asbestos (1332214) were examined. Lung tissue specimens obtained at autopsy from 55 deceased asbestos workers were examined by light and electron microscopy and energy dispersive X-ray analysis. Pleural plaques were found in the lungs of 33 subjects. They had been exposed primarily to chrysotile (12001295) for periods of 5 to 23 years, mean 18.1 years. Twelve subjects had been employed as miners and the other 21 as millers. Fifteen had asbestos bodies as well as pleural plaques. Among these, five had plaques of grade-I severity, seven grade-II severity, and three had plaques of grade-III severity which corresponded to plaque areas of less than 100, 100 to 300, or more than 300 square centimeters. The degree of plaque severity was not correlated with years exposure or number of asbestos bodies. The plaques usually consisted of bundles of collagenous fibers in a basket weave, concentric, or avascular arrangement. Mesothelial cell linings were occasionally seen on the plaque surface. The fiber bundles contained fibrocytic nuclei. A massive inflammatory infiltration of lymphocytes and plasma cells was frequently seen in the deeper portions or along the periphery of the plaques. Polarizing particles were found in intercollagenous fibers, of which many were found beneath the mesothelial lining. Many fibers contained silicon, magnesium, and iron. Others consisted primarily of silicon and calcium. The authors conclude that the severity of pleural plaques found in the lungs of asbestos workers is not related to the length of exposure or the occurrence of asbestos bodies. This suggests that plaque formation is governed primarily by individual susceptibility. Plaques apparently originate from beneath mesothelial cells and grow from their bases.