After 4-7 weeks of sublethal infection with human tubercle bacilli, the number of culturable bacteria recovered from each organ was greater in the neonatally thymectomized mice than in the controls. In the thymectomized mice the lesions of the lung and liver were extensive and exudative, and the epithelioid cell formation in the lesions were usually slight as compared with those in the control mice. The development of active protective immunity after BCG immunization was impaired to some extent in the neonatally thymectomized mice. Both in cases of infection with BCG and with a nonphotochromogenic strain of atypical mycobacteria, no significant difference between the neonatally thymectomized mice and the control mice was found in the number of culturable bacteria recovered from each organ during the observation period of 8-9 wks., although some differences were noted in the histologic findings of BCG-infected mice.