α1-Antitrypsin (A1AT) is a major protease inhibitor present in all mammalian sera that have thus far been investigated. A1AT is also highly polymorphic and is therefore a useful genetic marker. Previously reported A1AT polymorphism in domestic dogs consisted of two alleles designated as Pim and Pis which exhibited frequencies of 0.72 and 0.28, respectively, in a group of randomly collected mongrel dogs. North American species of Canis, which included gray wolves (n=29), Mexican wolves (n=20), coyotes (n=24), wolf-dog crosses (n=9), and red wolves (n=27) were tested for A1AT polymorphism. A1AT phenotypes were determined by isoelectric focusing, followed by direct immunoblotting using a specific antiserum. A1AT concentrations were determined by radial immunodiffusion. Concentrations of A1AT were similar to those found in domestic dogs (2.26 ± 0.3, ± SD mg/ml, ± SD) and tended to be higher in females than in males, possibly indicating that A1AT may be hormonally influenced in females. Three phenotypic band patterns were observed (M, MS, S). The allele frequencies for domestic dogs and gray wolves were very similar, 0.72 and 0.67 for Pim and 0.28 and 0.33 for Pis, respectively. The Mexican wolves had a significantly lower frequency of Pis = 0.10. Coyotes and red wolves were all found to be monomorphic for the Pis allele and were indistinguishable from each other in that respect.