Nitroblue tetrazolium dye reduction test is now widely used for early detection of bacterial infection. Practically nitroblue tetrazolium dye reduction test (N.B.T. test) gives certain clue to rule out any possibility of aseptic inflammation. Authors performed N.B.T. test for 80 orthopaedic diseases and 40 controls, and the results were as followings: 1. Forty-one of 60 subjects with bacterial infection had more than 10% formazan positive neutrophils, and 18 of 20 subjects without bacterial infection showed less than 10%. formazan positive neutrophils, The groups of the elevated N.B.T. responses were mainly the subjects who had osteomyelitis, soft tissue inflammation, pyogenic arthritis, and bone and joint tuberculosis. 2. The mean proportion of N.B.T. positive neutrophils was 3.8±2.78% in 40 controls, 4.7±4.23% in 20 nonifectious group 12.7±10.3% in 12 chronic osteomyelitis, 12.5±10.7% in 15 treated bone and joint tuberculosis, 20.9±15.65% in 15 untreated bone and joint tuberculosis and 20±10.70% in 11 pyogenic arthritis and 7 other soft inflammatory group. 3. In noninfectious group, 7 of 8 subjects with elevated ESR and 3 of 4 subjects with elevated body temperature revealed N.B.T. responses less than 10%. 4. In 23 subjects with the elevated N.B.T. responses as well as leukecytosis, 22 subjects were the groups of bacterial infection and rest 1 was noninfectious in nature. 5. Therefore, the N.B.T. test is found to be useful for the purposes of the early diagnostic aid of the bacterial infection and differential diagnosis between bacterial infections and other diseases.