Incidence and characterization of canine rheumatoid factor.

Rheumatoid factor (RF) was found in titers greater than 1:8 in 72.2% of cases of classical or definite canine rheumatoid arthritis (RA) and in 5.9% or normal sera. Serum fractionation and immunoabsorbant studies that much of the RF present was IgG, although activity was demonstrated in all 3 major immunoglobulin classes. Evidence of involvement of both IgG and IgM to form complexes of varying sizes was obtained.