Augmentation therapy with a plasma derived α1-Proteinase Inhibitor (α1-PI) has been demonstrated to be effective in restoring serum Alpha1-antitrypsin (AAT)* levels in individuals with AAT Deficiency (note: α1 PI and AAT are synonymous). The objective of this study was to demonstrate that the steady-state trough serum α1-PI levels, achieved by a new plasma derived α1-PI (Zemaira, study drug, ZLB Behring LLC, King of Prussia, Pennsylvania, USA), were bioequivalent to those achieved by the currently available α1-PI therapy, Prolastin (control drug, Bayer Corporation, Berkeley, California, USA), and maintained weekly trough serum antigenic α1-PI levels above the protective threshold of 11 μM. This multi-center, controlled study randomized a total of 44 subjects to receive either study or control drug for a 10-week double-blind phase. The control group was then crossed over to receive the study drug for the remainder of the study (14 weeks). The difference in mean trough serum antigenic α1-PI level between the treatment groups was 1.45 μM (90% CI-2.77, -0.13), signifying bioequivalence. The mean trough serum antigenic α1-PI level in the study drug group was greater than the therapeutic threshold of 11 μM, achieving a level of 17.7 μM during the steady-state period. Treatment-related adverse events (AEs) were seen in 7% and 21% of study and control drug treated subjects, respectively. No documented viral transmission occurred. These results demonstrate that the new plasma derived α1-PI (Zemaira) is bioequivalent to the currently available product Prolastin, is well tolerated, and safe with respect to the risk of viral transmission. Copyright © 2006 Taylor & Francis Group, LLC.