Subjects with primary immunodeficiency received modified immune serum globulin (IGIV) intravenously at various dose levels in long-term therapeutic studies. Therapy was effective and essentially free from adverse reactions. Two pertinent observations were made relating to the attained levels of serum IGG. Over a dose range of 100-225 mg/kg, the serum IgG level directly reflects the dosage administered. Sequential analysis of serum levels of IgG demonstrated three patient populations in 14 subjects receiving 150 mg/kg. The largest group, nine patients, had progressive reduction of serum IgG values compatible with the half-life of the reagent, with a return to the original serum IgG level in four weeks. A second population of four patients had a slower reduction of serum IgG over the four-week period. IgG values were significantly elevated over baseline values at the time of the next due infusion. In one subject serum IgG values varied greatly with rapid drops and elevations unrelated to the infusion.