In parallel with all the technical and organisational prevention measures taken in the chlor-alkali industry to further reduce the mercury emissions and protect the health of the workers (see Env. Prot. 11- Code of Practice - Mercury Housekeeping), it is necessary to organise a good biological monitoring programme (see HEALTH 2 - Code of Practice: Control of Worker Exposure to Mercury in the Chlor-Alkali Industry). Because it is non invasive and it reflects average exposure during the previous 2-4 months, measurement of mercury in urine is advised, although it is of limited use for detecting short term high exposures. This recommendation describes the methods developed for this monitoring, starting from the precautions to be taken for the sampling.