α₁-Proteinase inhibitor (α₁-PI) is a member of the serpin superfamily of serine proteinase inhibitors, which function in maintaining homeostasis through regulation of numerous proteolytic processes. In laboratory mice (Mus musculus domesticus), α₁-PI occurs in multiple isoforms encoded by a family of three to five genes that are polymorphic among inbred strains and that are located at the Serpina1 locus on chromosome 12. In the present study, we have characterized the α₁-PI gene family of inbred mice in more detail. We show that mice express seven isoforms, all of which are encoded by genes that map to the Serpina1 locus. In addition, polymorphism at the locus is defined by three haplotypes (Serpina1^b, Serpina1^c, and Serpina1^l) that differ with regard to both the number and identity of α₁-PI genes. Finally, we present the complete sequence of an 84-kb region of Serpina1 containing a tandem repeat of two α₁-PI genes.