The Pr protein, which is one of the major equine acidic prealbumins and which consists of a large number of phenotypes, has been studied with regard to its chemical identity. Serum samples of known Pr phenotype which had been treated with varying amounts of bovine trypsin were subjected to starch gel electrophoresis at pH 4.8. When a certain amount of trypsin was used, the Pr protein was markedly affected, whereas the other acidic prealbumins retained their normal electrophoreitic pattern. Extracts from three different regions of the acidic prealbumin field were tested by the casein precipitating inhibition test (CPI-test). Marked antitrypsin effect appeared against the extract from the Pr zone but not against the extracts from the two other acidic prealbumin zones. When acidic starch gel electrophoresis was combined with the CPI-test, a broad inhibitory zone appeared in the area of the Pr proteins. Pr protein was isolated by the use of agarose gel electrophoresis and sepharose column chromatography. The isolated protein which was tested for purity by gel electrophoresis had a molecular weight of about 60,000. It is concluded that the equine Pr protein corresponds to αi-antitrypsin.