In a 3-month period, three men who had worked for 5 to 19 years as welders or grinders of steel castings in a foundry acquired pneumonia caused by Acinetobacter calcoaceticus variety anitratus serotype 7J. Two of the men died, and postmortem examination showed mixed-dust pneumoconiosis with iron particles in the lungs. A calcoaceticus variety anitratus serotype 7J was isolated from the air in the foundry but the source was not found. The prevalence of antibody titers of 64 or greater to the 7J strain was significantly higher among foundry workers (15%) than among community controls (2%) (p less than 0.01). Sampling showed that the concentrations of total and metallic particles (especially iron) and of free silica in air inhaled by welders and grinders at the foundry frequently exceeded acceptable levels. These findings suggest that chronic exposure to such particles may increase susceptibility to infection by this organism, which rarely affects healthy people.