Total pulmonary nonasbestos mineral content was determined for a series of 20 patients who had no occupational dust exposure. To extract mineral fibers, lung tissue was dissolved in bleach and the treated sediment transferred to the electron microscope grid. Mineral fibers were identified using electron diffraction and energy dispersive x-ray spectroscopy. The mean number of nonasbestos fibers, 106 X 10(3)/g wet lung, was almost identical to the mean number of asbestos fibers, 102 X 10(3)/g. Thirteen different species or groups of nonasbestos minerals were found: apatite accounted for 18% of the total and talc for 16%. All other forms accounted for less than 8% each. Silica was found in every lung, and talc in 19 of 20 lungs. Of the fibers, 86% were shorter than 5 microns, and most of the fibers had aspect ratios less than 15. No correlations were seen between numbers or types of fibers and age, sex, or smoking. It is concluded that (1) substantial numbers of nonasbestos fibers are present in lungs of the general population. Most of these fibers are short and of low aspect ratio; and (2) by phase microscopy one is as likely to observe nonasbestos as asbestos fibers in these preparations, indicating that light microscopic methods are not suitable for this type of analysis. These data provide a baseline for comparison with patients believed to have a mineral fiber-related disease.