The results reported in this paper clearly indicate the release of osteal antigens of articular connective tissue in response to inflammatory alterations and injuries to the region immediately adjacent to an articulation. Further studies will have to be performed in an effort to find out whether these antigens are of pathological importance and if there is a relationship between the liberation of antigens and the intensity of inflammatory alterations or if the different and often difficult-to-interpret results of the determination of antibodies in the sera of patients with rheumatoid arthritis can be more easily explained by a simultaneous antigen analysis of the corresponding sera.