Although considerable progress has been made during the past two decades in the use of androgens to prevent attacks of hereditary angioedema, replacement of the deficient C1-inhibitor protein would provide a useful menas of treatment once an attack has begun. We studied the clinical use of C1 inhibitor that was partly purified on a large scale from pooled plasma. The in vivo efficacy and safety of this protein concentrate were evaluated during 11 intravenous infusions in eight patients with hereditary angioedema. Three patients received the C1-inhibitor preparation during an asymptomatic period. Increases in serum C4 activity provided evidence of the biologic activity of the infused inhibitor. Intravenous administration of the concentrate during acute abdominal or laryngeal attacks of hereditary angioedema in five patients resulted in abatement of symptoms in addition to increased serum C4 activity. No untoward effects of the intravenous administration of the C1 inhibitor were observed in these eight patients. Thus, this C1-inhibitor preparation seems to offer the potential for safe, effective replacement therapy and may provide a means of controlling an attack of hereditary angioedema that is in progress.