A survey of workers in seven man made mineral fibre (MMMF) production plants, the subject of a previous report, was conducted, with other blue collar workers serving as regional comparisons. Based on the median reading of chest radiographs by five readers, a low prevalence of small opacities, all at the 1/0 and 1/1 profusion levels, was again found: for workers with MMMFs, 23/1435 (1.6%); for comparison workers, 2/305 (0.7%). Spirometric measurements indicated generally healthy populations, and were not related to presence of opacities. Ninety three per cent (21/23) of MMMF workers with opacities worked at the two plants with the highest exposures to fine fibres, resulting in a dose-response relation across plants. For one location, the prevalences of opacities for the MMMF and comparison workers were not significantly different (5.9% (13/220) v 3.1% (2/65)). No comparison x ray films were obtained for the MMMF plant with the highest prevalence (6.6%), so a second phase of the study was conducted, with pre-employment films from these two plants. On this second reading, the prevalence of opacities was lower; there were no significant differences between the two groups of films, and no relation between opacities and exposure indices. There was considerable inter and intrareader variability. These results indicate no adverse clinical, functional or radiographic signs of effects of exposure to MMMFs in these workers.