Electrophoresis in polyacrylamide gels in the presence of sodium dodecyl sulfate was used to separate and quantitate the components of a washed immune precipitate. Serum was from patients with systemic lupus erythematosus known to have antibodies to soluble nuclear ribonucleoprotein (RNP) or to a soluble nuclear non-nucleic acid protein (Sm). Amounts of antibody that was predominantly IgG ranged from 0.2 to 8 mg/ml of patients' serum, and in some cases accounted for over 20% of the total serum IgG. Results demonstrate that some patients respond to the disease by producing large amounts of a specific antibody, and that these antibodies can contribute significantly to hypergammaglobulinemia.