In spite of the abundant literature on chronic nontuberculous arthritis, its etiology has remained obscure. The numerous authors who have classified this disease agree that it may be subdivided into two main types differentiated in the clinical and pathologic picture. One type is characterized by marked deformity and severe crippling and occurs usually within the first three decades of life. The other type has a far more insidious onset, rarely resulting in crippling, and usually occurs during the latter part of the fourth decade, or later. This paper deals with an experimental study of the first type, known as the proliferative type of Nichols and Richardson,1 the atrophic type of Goldthwait,2 the rheumatoid type of our British confreres, type I of Ely's classification,3 and as infectious or toxic arthritis by others. In the orthopaedic department of the Children's Hospital of Boston, during a period of several years, a number of