The synovial fluid beta1a (C3) and beta1e (C4) expressed by their ratio to corresponding serum concentrations) were studied in 31 cases of seropositive rheumatoid arthritis (RA+), 5 cases of seronegative rheumatoid arthritis (RA-1), and 15 cases of osteoarthrosis (OA) before osmic acid synoviorthesis on knees. This was repeated after synoviorthesis in the synovial fluid (SF) of 24 RA+, 4 RA-, and 10 OA patients. The following studies were undertaken: (a) the relationship between these components; (b) their correlation with the SF protein concentration and rheumatoid factor titre, when present. This analysis led us to the following conclusions. (1) Before synoviorthesis (a) The SF beta1e is significantly lower than beta1a in RA+. In OA, an inverse phenomenon is observed. (b) The concentration of beta1a and beta1e are proportional to the protein concentration in the SF of OA. A significant inverse relationship between beta1a, beta1e, and the titre of rheumatoid factor is found. (2) after synoviorthesis the same studies performed on knees at the time of one or more relapses shows that the same pathogenetic process is involved and that the immunological mechanism is little influenced by this treatment. In OA also the relapse differs very little from the initial process observed before synoviorthesis.