On April 26, 1951, a leproma developed in the subcutaneous tissue of the rat inoculated with the Fukuoka-strain of Myc. lepraemurium was homogenized in sterile physiological saline to make a 20 per cent solution. This suspension was divided into four aliquots and each aliquot was added respectively to an equal volume of physiological saline, 10% inactivated bovine serum water, 4% glycerine water, and Kirchner's medium containing 10% bovine serum. One ml of each mixed material was frozen, and then was dried by a rotary pump at room temperature. After drying these ampules were sealed and stored in a refrigerator at 4°C.
On April 26, 1961, the infectious activities of the bacilli in the ampules stored in the cold for ten years were determined by subcutaneous inoculation to mice. After about 5 months of inoculation, the mice were killed and the infectivities of the materials were determined by bacteriological examination.
The results obtained indicated that infective activity of Myc. lepraemurium could be maintained in vitro for even more than ten years by means of lyophilization, and that one of the most important factors influencing the maintainance of the infectivity of the bacilli seemed to be the dryness of the material at lyophilization, because the saline solution and serum water could be quickly dried and could maintaine the infectivity of the bacilli.