Cobalt, used as a binder for tungsten carbide, has been demonstrated to be toxic to the lung and to be a sensitizer of the bronchial airways among hard metal workers. To determine cobalt exposure levels in 9 job groups at a hard metal factory, 935 samples were collected from 356 workers. The arithmetic means of 8-hr time-weighted averages were more than 50 micrograms/m3 (American Conference of Governmental Industrial Hygienists' threshold limit value) in the powder preparation, rubber press, and shaping groups. The within-worker and between-worker components of exposure variation were estimated for each job group using the random effects analysis-of-variance model. The geometric standard deviations indicating the within-worker and between-worker components ranged from 1.88 to 2.77 and 1.00 to 2.31, respectively.