Two field trials were conducted in Malaya in which 75 volunteers were exposed in hyperendemic areas of scrub typhus. Thirty-four of these individuals received chloromycetin prophylactically for a total period of 3 weeks during and following exposure. They did not show clinical evidence of scrub typhus throughout the period of prophylaxis or the ensuing 5 days. However, scrub typhus developed in 15 volunteers in the prophylactic groups of the two trials 5 to 14 days after drug was discontinued. Although the attack rate among the volunteers in the two field trials varied markedly, there was no essential difference in the ultimate infection rates among the controls and those given prophylaxis in each test. Scrub typhus when it developed among volunteers in the prophylactic group was not significantly different from the disease in the controls except for the absence of eschar formation. Relapses were prominent features of the disease in the volunteers of both prophylactic and control groups. These had not been observed previously in untreated cases of scrub typhus or in naturally occurring cases which were treated with chloromycetin. Fifty-four per cent of the 37 persons in the two trials who contracted scrub typhus suffered one or more relapses. Various factors probably contributed to this phenomenon but the opinion is that the short course of chloromycetin therapy given very early in the illness probably was an important factor. Ten volunteers had received experimental scrub typhus vaccine during earlier investigations because of possible exposure to infection. The vaccination did not influence the incidence of infection or the course of the disease in those persons developing scrub typhus. Prolonged administration of chloromycetin as a prophylactic measure and its use in the treatment of the initial attacks of scrub typhus, as well as the relapses, indicated that the drug is of low toxicity for man, and that drug fast strains of Rickettsia tsutsugamushi are not readily produced.