It has been shown by several investigators (1, 2) that "BAL" (2,3-dimercaptopropanol) accelerates the excretion of arsenic in experimental animals poisoned with various arsenicals.In man also, the cutaneous application of BAL ointment (3) or the intramuscular injection of a solution in peanut oil and benzyl benzoate (4, 5) in cases of arsenical dermatitis sometimes results in an increased urinary arsenic excretion, as determined by analyses on 12-to 24-hour specimens collected before and after its administration. In view of the short-lived effect of BAL in experimental animals, it is possible that these effects of BAL on urinary arsenic excretion in man would have been more prQnounced and more regular had it been possible to obtain urine specimens at 2-to 4-hour intervals.The present paper represents an attempt to determine the effect of BAL on the normal urinary arsenic excretion in man, as compared with its effect in human subjects exposed for brief intervals to minimal concentrations of an arsenical smoke (diphenylcyanoarsine). By treating the latter group at varying intervals after exposure and by obtaining urine specimens at 2-hour intervals, it was hoped to delimit somewhat more precisely its effect on the urinary excretion of arsenic, and to ascertain whether BAL could be used to confirm suspected minimal exposure to arsenical poison gases. The results might be of significance also in relation to the detection and prevention of industrial arsenic poisoning. METHODS 1. Normal controls.Six normal young soldiers served -as controls. On the morning of the test, after breakfast, bladders were emp-1, 2 Capt., M.C.tied at 8 a.m. No cigarettes or solid food were allowed during the following 12-hour experimental period. Two hundred ml. of H20 were thereafter taken by mouth each hour, and 500 ml. of milk at 12 m., and again at 5 p.m. Urine specimens were collected every 2 hours into Erlenmeyer flasks cleaned with arsenic-free acid. At the end of the third 2-hour period, each man was injected intramuscularly with 3.5 mgm. of BAL per kgm. body weight, administered as a 10 per cent solution in peanut oil with 20 per cent benzyl benzoate as a solubilizing agent (2). Following treatment, the urine was again collected every 2 hours for 6 hours. There were thus a total of 6 2-hour specimens on each volunteer, 3 before and 3 after the injection of BAL.The arsenic content of the individual specimens was determined by the method of Magnuson and Watson (6). The milk was analyzed and found to be arsenic-free (less than 0.01 micrograms per ml.).2. Exposure of men to diphenylcyanoarsine (D.C.) smoke.Twelve men were exposed in a gas chamber for 6 minutes to diphenylcyanoarsine smoke, at a total arsenic concentration of 3.9 mgm. per cu.m. Of this, 1.6 mgm. was present as diphenylcyanoarsine, 0.5 mgm. as other forms of organic arsenic, and 1.8 mgm. as inorganic arsenic.The exposed men were then divided into 4 groups of 3 men each. Studies similar to those carried out on the 6 normal controls were carried out on one group of 3 men...