An extract of human lymphocytes from continous cell culture was used as the antigen source to detect antibodies in sera of patients with Sjögren's syndrome (SS). Using double diffusion in agarose, 85 per cent of a selected group of patients had precepating antibodies. Three precipitating antigen-antibody systems were detected and were shown to be different from those described previously in othe systemic rheumatic diseases. The SS precipitating antibodies were temporarily classified as precipitins, A, B, and C. SS patinets with sicca syndrome but without clinical rheumatoid arthritis had precipitin systems A and/or B, and SS patients with associated rheumatoid arthritis had precipitin system C. Serum reactants were demonstrated by immuno-electrophoresis to migrate in the gamma globulin region. The precipitating activity of the serum factors was not destroyed by treatment with 2-mercaptoethanol and was not removed by absorption of rheumatoid factor from the sera. The reactivity of the lympuocyte antigens was destroyed by treatment with trypsin but not by deoxyribonuclease or ribonuclease.