Tubercle bacilli were injected intravenously to mice after dusts were injected into their subcutaneous tissue. Out of 23 kinds of dusts tested, a quartz dust enhanced most the growth of mycobacteria injected intravenously in mice. Based on this a mixture of the sputum from patients with pulmonary tuberculosis and finely powdered crystalline silica was inoculated subcutaneously into the flanks of mice. Growth of tubercle bacilli was demonstrated occasionally for the sputum specimens from smear-positive and culture-negative cases, and even for smear- and culture negative cases. When a sputum specimen which yielded many colonies on routine egg medium were tested, the growth of bacteria became visible as early as 2 weeks after inoculation. On the other hand, it took 3 to 4 weeks for the visible growth of bacilli, when a sputum specimen yielded only a few or no colonies on culture medium. In this method, the bacilli appeared earlier and more frequently than in the routine culture method on egg medium. Thus an inoculation into mice with a mixture of the sputum and quartz powder was shown to be a sensitive method for the diagnosis of pulmonary tuberculosis.