Alpha-1-antitrypsin (α1-AT) is a member of the serine protease inhibitor family regulating numerous proteolytic processes. The genetic disorder, α1-AT deficiency, is well known as a cause of hereditary pulmonary emphysema and liver cirrhosis. To create an animal model of human α1-AT deficiency, we disrupted the major murine isoform PI2, which is similar to human α1-AT and is one of 7 α1-AT isoforms found in the mouse. The ability of the serum to inhibit the activities of human leukocyte elastase (HLE) and human chymotrypsin (CYT) was significantly lower in heterozygous mice (α1-AT/PI2-/+) than wildtype (α1-AT/PI2+/+) mice (73.2% vs. 100% for HLE and 67.8% vs. 100% for CYT, respectively; P< 0.05). The distribution of genotypes among F2 progeny was not in accordance with Mendelian distribution (P< 0.01), as the percentages of wild-type, heterozygotes and homozygotes were 47.8%, 37.3% and 14.9%, respectively. Thus, it is likely that impairment of the protease inhibitor had a critical effect on fetus development. The α1-AT/PI2 deficient mouse will be a useful animal model for elucidating the function of α1-AT in fetal development, studying the mechanisms of chronic inflammatory disease and evaluating therapeutic candidates for the treatment of inflammatory disease.