The amount of extravascular immunoglobulin deposits in the synovial membrane of patients with osteoarthritis and rheumatoid arthritis was studied in comparison with that of patients suffering from non-joint diseases. Immunoglobulin deposits were immunostained using the three-layer immunoperoxidase method. The staining results were quantified with the help of a microscope photometer connected with a scanning stage. Several experiments involving artificial test substrates, diseases with allegedly increased extravascular deposits or diseases not exhibiting extravascular deposits of immunoglobulins are used to validate the described microspectrophotometrical approach for measuring extravascular immunoglobulin deposits. The scanning photometry demonstrates significantly higher amount of extravascular immunoglobulin deposits in rheumatoid arthritis as compared with osteoarthritis and non-joint diseases.