Cobalt urinary excretion should be a good way to know the level of exposition of subjects submitted to hard metals. The study is done on 28 workers of 3 different workrooms. Sunday 24 hours diuresis is retained. For 5 workers, the urinary cobalt measurements are done from samples made at the beginning and the end of work during 5 days. Atmospheric dust measurements are realized in different points of the 3 workrooms (individual and general samples). For measures atomic absorption spectrometry is used. The results show a good correlation between cobalt exposure and urinary concentrations. Friday evening urinary cobalt is a good expression of week exposure. The results obtained well correlate with those of others authors. Cobalt urinary measures are proposed for supervision of workers exposed to hard metals.