Sheep plasma proteinase inhibitor, analogous to human alpha 1-proteinase inhibitor (alpha 1 PI), was isolated to homogeneity. Purification was achieved by using (NH4)2SO4 precipitation, concanavalin A-Sepharose chromatography, Mono Q ion-exchange chromatography and PAGE. Sheep alpha 1 PI had an Mr of 56,000, inhibited human leucocyte elastase, pig pancreatic elastase and bovine trypsin on a 1:1 molar basis and had a plasma concentration of 1.6 +/- 0.21 g/l (mean +/- S.D.). Amino acid/carbohydrate composition (15% glycosylated) was similar to that of human alpha 1 PI (16% glycosylated); N-terminal analysis to 31 residues revealed 48-52% identity between the human and sheep proteins. Sheep alpha 1 PI was susceptible to oxidative inactivation by chloramine-T. Re-activation with the use of methionine sulphoxide peptide reductase and dithiothreitol indicated the presence of a methionine residue at the active site. These results establish that sheep alpha 1 PI has functional and structural characteristics close to those of human alpha 1 PI.