Cobalt is considered to be the chief component in the tungsten carbide mixture responsible for hard-metal disease. An animal inhalation study was undertaken simulating conditions under which these workers in the tungsten carbide industry are exposed to cobalt. In this semichronic study, miniature swine were exposed to an inhalation of pure cobalt metal powder at concentrations of 0.1 mg/m3 (present TLV) and 1.0 mg/m3. At periodic intervals, the following studies were performed: pulmonary function tests, electrocardiograms, x-ray examinations, blood and urine tests, electron microscopy, and light microscopic pathology. Early detection of pulmonary disease is apparent from the pulmonary function tests showing a mark decrease in lung compliance, and from electron microscopy showing an increase in the amount of septal collagen. This study demonstrates that the present TLV of 0.1 mg/m3 for cobalt seems to be too high, even at this relatively brief duration of exposure.