Collagen-free extracts were prepared from bovine, porcine and canine hyaline, elastic and fibrous cartilages, articular capsule, tendon, aorta, cortical bone and regenerating articular surfaces. The extracts were investigated with antisera to bovine nasal septal cartilage, dog articular cartilage and non-collagenous protein fraction of bovine cortical bone. Immunodiffusion, immunoelectrophoresis, and immunohistochemical methods were used. In the different supporting tissues of the three animal species a common antigen, probably of proteoglycan origin, was demonstrated. The finer differences in antigenicity between the different tissues are probably due to the variations in proteoglycan composition of the given supporting tissues. Owing to the wide-spread occurrence of the antigen, the authors suggest the term "species-common connective tissue antigen" instead of the "species-common cartilage antigen" used so far.