To investigate the hazards of exposure to levels of elemental mercury lower than the present TLV value (0.05 mg/m3) a group of dental workers and a nonexposed group were studied. The amount of mercury in blood from mothers and babies at the time of delivery was similar. The exposed group had increased mercury content in placenta and foetal membranes. Mercury accumulation in these organs might serve as a protection for the foetus against mercury exposure. Since exposure to sub-TLV concentrations of elemental mercury during pregnancy will not be reflected in blood mercury content at the time of delivery, the amount of mercury in placenta and foetal membranes might serve as a biological indicator of such exposure.