The correlations between airborne concentration of mercury found and the clinical and biological manifestations in workers engaged in the manual manu-facture of mercury glass bubble relays has been studied. The workplace sampled was a small room in which there were five workers. Airborne mercury sampling was performed by using charcoal tubes and 3M mercury vapor monitors. Whole blood mercury concentration (HgB), ALA-D activity, free erythrocyte porphyrin IX (FEP), and the urinary output of mercury (HgU) were determined for each worker. Clinical examination revealed a tremor in the hands, a metallic taste and headaches. Workers with HgU higher than 400 μg/L were orally treated with N-acetyl-d, l-penicillamine, and temporary removal of them from exposure was re-commended. A very good linear correlation of environmental concentrations of mercury as time weighted average (TWA) between charcoal tubes and 3M monitors was found. The regression analysis for the results obtained as TWA and the biological parameters measured fit a logarithmic equation well, as well as the values obtained for HgB, ALA-D and HgU when they were compared.