Inhalation of particulate cobalt has been known to induce interstitial lung disease. There is growing evidence that apoptosis plays a crucial role in physiological and pathological settings and that the ubiquitin-proteasome system is involved in the regulation of apoptosis. Cadmium, the same transitional heavy metal as cobalt, has been reported to accumulate ubiquitinated proteins in neuronal cells. On the basis of these findings, we hypothesized that cobalt would induce apoptosis in the lung by disturbance of the ubiquitin-proteasome pathway. To evaluate this, we exposed U-937 cells and human alveolar macrophages (AMs) to cobalt chloride (CoCl₂) and examined their apoptosis by DNA fragmentation assay, 4',6-diamidino-2'-phenylindol dihydrochloride staining, and Western blot analysis. CoCl₂ induced apoptosis and accumulated ubiquitinated proteins. Exposure to CoCl₂ inhibited proteasome activity in U-937 cells. Cobalt-induced apoptosis was mediated via mitochondrial pathway because CoCl₂ released cytochrome c from mitochondria. These results suggest that cobalt-induced apoptosis of AMs may be one of the mechanisms for cobalt-induced lung injury and that the accumulation of ubiquitinated proteins might be involved in this apoptotic process.