The uninary hippuric acid (H.A.) of workers in an automobile factory, a plastic toy factory, and a gravure painting factory, where toluene is used, was determined, and the urinary hippuric acid and the methyl hippuric acid (M.H.A.) of the worker using thinner containing toluene and xylene were also measured. The concentration of toluene or xylene in the air was determined by gas chromatography with FID and by a Kitagawa's detector, and the average concentration inhaled by workers was calculated with a chart showing the relationship between the toluene or xylene density in the air of an artificial exposure chamber and the excretion per minute or the concentration of H.A. or M.H.A. of volunteers as reported previously. The results obtained were as follows. 1. Seasonal measurements of urinary H.A. and M.H.A. in workers, who were using thinner containing toluene and xylene, were conducted six times in two years. Both urinary excretions per mimute of H.A. and M.H.A. increased during the working time. As daily urinary H.A. and M.H.A. excretions varied according to the concentration of toluene or xylene due to the kind and velocity of the spraying work; in fact, the amount of excretion of worker's urinary H.A. and M.H.A. during work varied every time when the survey was made. Therefore, it was found necessary to check the excretion of H.A. and M.H.A. in worker's urine at least once a month. 2. Molar ratio of toluene to m-and p-xylene in the thinner was similar to that of urinary H.A. and to M.H.A. in the worker who was using that thinner. 3. We can screen out a worker exposed to toluene plus xylene at 200 p.p.m. from the excretion perminute by the following equation. (H.A. excretion per minute in working time (mg/min))/(H.A. excretion exposed to 200 p.p.m. of toluene (3.77 mg/min))+(M.H.A. excretion per minute (mg/min))/(M.H.A. excretion exposed to 200 p.p.m. of xylene (4.10 mg/min)) ⪌ 1…(1) or simply by (H.A. + M.H.A. excretion mg per minute (mg/min))/(3.95 mg/min) ⪌ 1…(2) When the value of equation (1) or (2) is over 1, the average concentration of the sum of toluene and xylene in the worker who has inhaled both, is judged to be over the threshold limit value (T.L.V.). Equation (2) was found useful in the case in which the sum of H.A. and M.H.A. exretions per minute was measured by the simple paper chromatograph with buthanol-acetic acid-water as a developing solvent. 4. The worker working in the atmosphere higher in the density of toluene excreted so much higher concentration of urinary hippuric acid. And also, the higher the densities of toluene and xylene in the environmental air, the higher was the concentration of glycine conjugates (H.A. + M.H.A.). 5. In evaluating the results of the medical examination of workers who were using thinner containing toluene and xylene, results of measurements of urinary H.A. and M.H.A. of workers, indicating the amount of toluene and xylene inhaled by the workers, should be taken into account in addition to counting erythrocytes and leucocytes, because toluene and xylene have relatively higher narcotic actions.