A study of X-ray findings in tremolite (14567738)/actinolite (1332214) exposed workers was conducted. X-ray films of 184 males who had been employed at a vermiculite (1318009) mine (SIC-1499) and mill near Libby, Montana, for at least 5 years from 1975 to 1982 were read and scored according to criteria of the International Labor Organization. Company administered questionnaires on smoking habits and respiratory symptoms were reviewed. Individual cumulative fiber exposure estimates in fiber/years (f/yr), tenure, and age were calculated to the date of the most recent radiographic examination. The prevalence of X-ray abnormalities was as follows: small opacities, 10 percent; any pleural change, 15 percent; pleural calcification, 4 percent; and pleural thickening on the wall, 13 percent. F/yr exposures were significantly related to small opacities, any pleural change, and pleural thickening on the wall, but not to pleural calcification. The association between f/yr, small opacities, and pleural change was not significant under joint control for cigarette smoking and age. Few cases of small opacities, however, were found among nonsmokers and younger workers with low f/yr exposure. The exposure response relationship for small opacities was also investigated using data from other studies, in which subjects had no dust exposure. The prevalence of small opacities was significantly higher for vermiculite workers with more than 100f/yr exposures compared to workers in non dusty occupations. The authors conclude that an 8 hour time weighted fiber exposure of approximately 50 years, is not a conservative standard for preventing X-ray evidence of small opacities.