In performing MAC chemotherapy, we are faced with the following two problems: first, unlike for Mycobacterium tuberculosis, we have neither any suitable bacteriocidal drugs nor bacteriocidal regimens for MAC treatment; secondly, in case of MAC, unlike in the case of Mycobacterium tuberculosis, in vitro sensitivity does not correlate with in vivo sensitivity. For these reasons, we find difficulty in planning a rational protocol for MAC treatment, and thus depend on previous experiences of other physicians in MAC chemotherapy. Here, we have tried to evaluate such previous experiences objectively, and studied retrospectively the effects of previous combination chemotherapy cases of pulmonary MAC infections at National Tokyo Hospital. We selected 170 cases which had received the same chemotherapy continuously over a period of six months. Concentration of bacilli cultured, 8 weeks in Ogawa solid egg containing medium was translated semi-quantitatively to colony forming units (CFU) according to the Japanese guideline for acid fast bacilli test. Having set a mean CFU of 3 times sputum culture before treatment as 100%, we calculated a six month sequential bacillary response to a regimen and plotted the bacillary response curves. The response curve of the total 170 MAC treatment cases exhibits a minimum point of 42.9% at 2 months, and subsequently rises to 71.1% after six months compared to the state before treatment. The response curves of various regimens of multidrug chemotherapy indicate that combinations of more than 3 drugs including aminoglycoside and clarithromycin are most effective. However, although some effectiveness is indicated, neither the present drugs nor regimens are capable to achieve a bacteriocidal effect in MAC treatment.