The histopathological (H. E., V. G., PAS-Alcian, Safranine 0, Gömöri) and electron-microscopical investigations were carried out on twenty samples of articular cartilage taken during operations from patients with Rheumatoid Arthritis (R. A.) and from others with traumatism, as controls. Histopathologically, the rheumatoid synovial membrane is characterized by synovitis with abundant perivascular lymphoplasmocytic infiltrates. At the pannus synovia-cartilage junction we found the invasive and destructive inflammatory infiltrates penetrating and eroding the cartilage. The histopathological characteristics of the rheumatoid articular cartilage lie in alteration of tinctorial activity, affection of reticuline collagen network and the presence of superficial and deep cartilaginous fissures. The histopathological alterations were confirmed ultrastructurally. Immunologically we found pathological serum values regarding the immune circulating complexes (I. C. C.) (mean = 104 +/- 1.04 U), anticollagen II antibodies (mean = 538 +/- 5 U), reactive Protein C (mean = 16.75 +/- 1.95 mg%) and orosomucoid (mean = 151.1 +/- 4.91 mg%), in seropositive R. A. The corroboration of histopathological, electronmicroscopical and immunological data show the inflammatory and autoimmune feature of this rheumatic disease.