INTRODUCTION This study was made on the material collected during the treatment of fifty cases of bone tuberculosis. The patients were under treatment on the surgical service of the Barnes Hospital and the St. Louis Children's Hospital. The material obtained at operation was investigated in the laboratory of surgical pathology of the Washington University School of Medicine. From the clinical standpoint, the cases used presented varying types of those manifestations usually associated with infectious bone lesions. Forty-two of the patients were adults; eight were children. In most cases, the symptoms and physical data made diagnosis of the tubercle bacillus as the etiologic factor a matter of no great difficulty. In some cases, diagnosis of tuberculosis was not made certain until the material placed in our hands by operation was sectioned and studied microscopically, and the confirmative evidence of animal inoculation was received. The clinical confusion between lesions due to syphilis