A therapeutic and prophylactic effect of α-mercaptopropionyl glycine for toxicity of mercury and its compounds was tested in comparison with the effect of D-penicillamine. The nine subjects working in an industrial factory manufacturing insecticides made from mercury were selected for the investigation. The daily dosis of α-mercaptopropionyl glycine was 450mg and it was 1000mg of D-penicillamine. The results obtaind were as follows: 1. The average daily excretion of mercury in urine was increased 1.4 to 1.6 times (average, 1.5 times) of the amount estimated before the administration when the subjects were in exposure of mercury. The increase was more than that obtained with the use of D-penicillamine (average, 1.2 times). 2. The average daily excretion of mercury in urine was increased 2 to 3 times of the amount estimated before the administration when the urinary excretion of mercury was determined after the exposure. The increase was lasted as long as 12 days after the discontinuation of the exposure. α-Mercaptopropionyl glycine was proved to be more effective than D-penicillamine when it was tested serially in a same subject. 3. From the results obtained here, it was concluded that the administration of α-mercap-topropionyl glycine at the daily dosis of 450mg was proved to be effective for the prophylaxis of mercury toxicity. It was especially true when the sujects were working in the exposure of mercury, therefore, it was recommended to use α-mercaptopropionyl glycine as long as the increase of urinary excretion of mercury was no more observed. 4. During the administration, there observed no appreciable side effects, and so the prophylactic use of α-mercaptopropionyl glycine was proved to be not hazardous.