The A. describes the normal pathological proteic, immunological and cito-immunological features of synovial fluid. According to these features, the synovial fluid have been classified into 3 groups: a) non-inflammatory, animmune fluids (without pathological immunitary features); b) inflammatory, normoimmune fluids (having immunological features with normal physiopathological reactions); c) inflammatory, iperimmune-disimmune fluids (with complex and abnormal immunological features). Histologically, they correspond to: a) non flogistic artrosynovitis (synoviosis, simple synovitis); b) inflammatory arthrosynovitis without tipical features (they are also described as cronic aspecific arthorosynovitis) or characterized by pathohomonic features (tubercolar synovitis, gouty synovitis. ecc.); c) rheumatoid arthrosynovitis (and inflammatory arthrosinovitis rheumatoid like). There is a generical correlation between histological aspects of groups a) and b) and humoral values of synovial fluid. Tipical correlations have been found between histological features of RA and humoral values of synovial fluid. These are expression of individual immunological reactivity of tissues in presence of rheumatoid factors. The tissutal reactivity can subiecy even to endogenous or exogenous causes.