GM-CSF binding to serum of patients treated with IgG. Two patients with systemic lupus erythematosus and mixed connective tissue disease, respectively, received 60 g of either Gammagard or Sandoglobulin over 3 days at intervals of 1 month. Sera were collected immediately before and after three consecutive series of infusions. Data are means of duplicates and expressed as percent saturable binding of ¹²⁵I-GM-CSF to IgG relative to the total amount of tracer using 4% (vol/vol) serum and 2,500 cpm/100 μL of¹²⁵I-GM-CSF.