Epidemiological studies suggest an association between excess lung cancer risk and work in ferrous foundries. No causative factors have been identified. The foundry environment is complex, and little is known about the health effects of the levels of known or suspected carcinogens present. As a part of an effort to identify causative factors and to assess current risk, an Ames assay of particulate mutagenicity was undertaken. Mutagenic levels were found to exceed those of urban air and distribution findings were consistent with those of a previous epidemiological study that identified high- and low-risk areas within the foundry. The biological significance of these mutagenic levels is unknown but is under study. A mechanism is suggested to explain current and historical findings.