Advantages of biological monitoring in the assessment of human exposure to chemicals in the workplace have been recognized by the American Conference of Governmental Industrial Hygienists (ACGIH). In 1982, the ACGIH established a committee to develop documentation and recommendations for biological exposure indexes (BEIs) that could be used with threshold limit values (TLVs) for better protection of worker health. A list was presented of organic agents, from pesticides to solvents, that have had biological index values recommended by European countries and the World Health Organization. Evolution and use of BEIs and their relationship to TLVs were discussed. Human workplace and experimental studies were reviewed, and such factors as absorption, metabolism, elimination, and pharmacokinetics were considered. Sampling instructions, methods of analysis, and storage stability were reviewed. The BEI is intended to be directly correlated to the TLV time weighted average when samples are collected as stated in the documentation and when the worker is doing moderate work. The BEIs that have been approved by the ACGIH Board of Directors for inclusion in the 1985 to 1986 edition of the TLVs booklet are listed in table form. Documentations have been published for carbon-monoxide (630080), ethylbenzene (100414), styrene (100425), toluene (108883), trichloroethylene (79016), benzene (108952), and xylene (1330207). The present committee will be expanded, and documentations on seven additional chemicals are in preparation.