The concept of the biological exposure index (BEI), developed by the American Conference of Governmental Industrial Hygienists (ACGIH) for assessing exposures to chemicals in the workplace, was reviewed. The BEI was defined as an index chemical that appears in a biological fluid or in expired air following exposure to a workplace chemical, and which serves as a warning of exposure. Application of the BEI was illustrated by four NIOSH studies. Breathing zone and blood samples monitored in a study of carbon-monoxide (630080) (CO) in a warehouse showed postshift levels of blood carboxyhemoglobin (COHb) for nonsmokers were above 5 percent and those of smokers were above 10 percent. The BEI established after this study was "less than 8 percent COHb". A degreasing operation using trichloroethylene (79016) (TCE) was monitored by breathing zone and urine sampling. The environmental measurements ranged from 200 to 420mg/m3 TCE, compared to the NIOSH recommended limit of 270mg/m3, with the conclusion that TCE exposure was excessive. However, the levels of urinary TCE metabolites were only about half of the subsequently established BEI for TCE, illustrating a situation in which biological monitoring and environmental data do not agree. Preliminary data were presented from a third study, on exposure of shipyard painters to 2-ethoxyethanol (110805). In a fourth study, experimental exposure of human volunteers to acetone (67641) and methyl-ethyl-ketone (78933) (MEK) indicated that workers exposed to these solvents would accumulate acetone over the workweek, while MEK would be totally eliminated before the next day. A companion experiment with toluene (108883) yielded a blood half time of toluene between that of acetone and MEK, but with little toluene remaining the day following exposure. Major areas of future biological monitoring research were discussed.