We performed serologic and synovial investigations in rheumatoid (Latex 1/1280, 1/640, negative and Waaler Rose 1/1024, 1/512, negative), non-rheumatoid and control lots. The immunological results were correlated with ultrastructural changes found in the synovial fluid (SF) at the same titres of rheumatoid factor (RF). The pathologic values of the circulating immune complexes (CIC) (mean = 108.05 U), IgM (mean = 420 UI/ml), IgG (mean = 355.36 UI/ml), and anti-collagen II antibodies (mean = 558.6 U) were present at high titres of RF (Latex 1/1280, Waaler Rose 1/1024). These cases had also major ultrastructural changes of the nucleus and cytoplasm. We inferred from this the implication of the immune factors in the etiology and pathology of the Rheumatoid Arthritis (RA). The high, titres of RF were correlated with pathologic values of the C-reactive-protein (CRP) (mean = 13.31 mg%) and alpha-1-acid glycoprotein (A-1-GA) (mean = 158.3 mg%). The decline of the complement fraction C3 from the synovial fluid in RA confirms the immune character of the rheumatoid synovitis and may be useful in the diagnosis process. The significantly lower concentrations of the protease inhibitors alpha-1-anti-trypsin (A-1-AT) (mean = 165.1 mg%) and alpha-2-macroglobulin (A-2-M) (mean = 129.6 mg%) in synovial fluid suggest a diminution of the anti-proteasic activity due to local immune conflict.