The functional affinity of IgM rheumatoid factors (RF) was measured in 31 patients with rheumatoid arthritis (RA), 24 with systemic lupus erythematosus (SLE), 13 with Sjögren's syndrome (SS), and in 13 seropositive healthy individuals. The functional affinity of IgM RF from patients with RA was significantly lower than in the other clinical groups studied. In addition, there was a significant inverse correlation between functional affinity and titre of IgM RF in all the groups. These results suggest that the usual mechanisms of affinity based selective pressure (somatic diversification and antigen selection) may operate differently for autoantibodies to serum antigens such as IgG.