Biological monitoring tests were conducted in humans experimentally exposed to methyl-ethyl-ketone (78933) (MEK) and toluene (108883), both alone and in combination, by inhalation. Subjects were exposed in a 2 cubic meter exposure chamber for 4 hours to 200 parts per million (ppm) MEK (20 subjects), 100ppm toluene (30 subjects), 100ppm MEK plus 50ppm toluene (16 subjects) or, as a positive control, 0.8 milliliters ethanol per kilogram of body weight (18 subjects). Chemical concentrations were measured by infrared spectrometry. Alveolar breath samples were collected during exposure, at the end of exposure and 1.5 hours postexposure. Venous blood samples were drawn from ethanol and MEK exposed subjects after 2 and 4 hours of exposure, and from MEK plus toluene exposed subjects after 2 hours of exposure and 1.5 hours postexposure. Samples were analyzed by gas chromatography. The average toluene concentrations in the breath of subjects exposed to toluene alone were 4.8, 7.0 and 7.6ppm after 1, 2 and 4 hours of exposure, respectively, and 2.4ppm 1.5 hours postexposure. The corresponding MEK average breath concentrations were 5.1, 7.7, 9.1 and 1.8ppm. In subjects simultaneously exposed to MEK and toluene, average breath concentrations after 2 and 4 hours of exposure and 1.5 hours postexposure were 4.0, 4.2 and 0.4ppm, respectively, for MEK and 2.2, 3.0 and 0.7ppm, respectively, for toluene. The authors note that these levels do not exceed those for single exposures, and conclude that the OSHA mixtures formula, which assumes additivity, is conservative in relation to the exposure conditions studied.