The American Conference of Governmental Industrial Hygienists (ACGIH) has recognized the value of biological monitoring for assessing worker exposure to chemicals. The ACGIH has established a Biological Exposure Indices Committee to review literature, prepare documentation analogous to the widely used TLVs, and recommend biological exposure indices (BEIs) for selected workplace chemicals. The BEIs represent an in-depth review of the literature and address such important issues as routes of exposure, absorption, metabolism, uptake, pharmacokinetics, sampling, methods of analysis, interferences (both chemical and metabolic), and interpretation of results. The recommended BEIs represent levels of some biological parameter that would be found in a worker following an 8-hour exposure (at moderate work) to the current TLV. To date, ten documentations and an introduction have been published by the ACGIH. They include toluene, xylene, ethylbenzene, carbon monoxide, styrene, benzene, n-hexane, lead, phenol, and trichloroethylene. Others are in preparation.