A patient with acquired agammaglobulinemia was treated with plasmapheresis. The rationale for this procedure was based on the presence of a cytotoxic autoantibody with specificity for helper (TH2-) T lymphocytes. Plasmapheresis reduced the autoantibody concentration to undetectable levels, which resulted in an increase number of helper T cells. These T cells provided normal in vitro helper activity. Plasmapheresis did not correct a concomitant suppressor T-cell defect, and the clinical remission ended during the fifth month of exchange therapy.