The effect of gamma_rays upon Mycobacterium tuberculosis and Mycobacterium rnarianum was investigated. The bacteria were irradiated with ± 10.000 r to ± 100.000 r. Irradiations of Mycobacterium and Mycobacterium marianum with gamma_rays did alter neither the mor_phology, the biochemical properties nor the resistence to Streptomycin, PAS and Isoniazid. Mycobacterium marianum showed to be more resistent to gamma_rays than Mycobacterium tuberculosis. Higher doses of irradiation of the arganism resulted in the later and fewer appearance of the colonies on Lowenstein_Jensen medium. 4 days old culture of tubercle bacilli on Lowenstein_Jensen seemed to be killed by irradiation with ± 70.000 r. Irradiation of suspended Mycobacterium tuberculosis with ± 30 .000 r did not inhibit the growth of the tubercle bacilli on Lowenstein_Jensen medium if cultured directly after irradia_tion. On the other hand, culturing the bacillary suspension 5 days after irradiation with ± 30.000r on Lowenstein_Jensen medium no growth would be seen. 4 days' old culture or suspension of Mycobacterium marianum was not inhibited in its growth by irradiation with ± 100.000 r. Irradiation with ± 90.000 r of a bacillary suspension of Mycobacterium marianum cultured 5 days later failed to show growth on Lowenstein_Jensen medium. The morphology, biochemical activities and resistance to Streptomycin, PAS and Isoniazid of the first to the fifth generation of M. tuberculosis and M. marianum irradiated by gamma rays remained unchanged as found in the parent strain. TAN THIAM HOK dan F. SUHADI; Bagian Mikrobiologi, Fakultas Kedokteran Universitas Indonesia dan Pusat Penelitian Pasar Jum'at, Badan Tenaga Atom Nasional.