A field study on the metabolites of methyl isobutyl ketone (MIBK) in the urine of 20 workers who were exposed to mixed solvents containing MIBK, and in that of a subject exposed to pure MIBK with 3 separate control subjects were observed. One of the urinary metabolites, 4-methyl-2-pentanol (4M2P) was identified by gas chromatography-mass spectrometry only in the urine of workers exposed to MIBK. Time-weighted average concentration of the mixed solvents for 8 hours were as follows ; MIBK 21.9±15.1 ppm (m±SD), toluene 18.3±8.9 ppm, ethyl benzene 9.6±4.6 ppm, xylene (total) 18.9±12.5 ppm. The regression equation between MIBK concentration X (ppm) in air and 4M2P concentration Y (mg/g creatinine) in the urine was : Y=25X-10.25. Its correlation coefficient was 0.63. Thus the urinary concentration of 4M2P corresponding TLV (Threshould Limit Values : 50ppm) of MIBK was 2.61 (mg/g creatinine). In a subject exposed to pure MIBK, the time-weighted average concentration of MIBK for 6 hours was 42.3ppm and 4M2P concentration in the urine was 0.42 (mg/g creatinine).