Antithrombin III (AT III) was isolated by two procedures using polyethylene glycol-400 (PEG) precipitation as the first stage. The PEG supernatant (PEG-sup) was applied to a heparin-affinity chromatographic system and AT III-heparin cofactor (AT III-HCF) was isolated. The PEG precipitate (PEG-ppt) was separated by a Sephacryl S-200 column. Fractions were collected and those demonstrating maximum AT III antigen and progressive thrombin inhibition were pooled and reapplied to the washed Sephacryl S-200 column. Fractions were again collected and assayed via specific antisera for AT III, alpha 1-antitrypsin (alpha 1 AG), alpha 2-macroglobulin (alpha 2 M), and alpha 1-acid glycoprotein (alpha 1 AT). AT III antigen (AT III AGN) and progressive function were confined primarily to one peak containing virtually no alpha 2 M, a low level of alpha 1 AT, and moderate quantities of alpha 1 AG. The PEG-sup, PEG-ppt, AT III-HCF, and the fraction obtained after two passes across Sephacryl S-200 (S#2) were similar in that they showed reactivity with specific AT III antisera and demonstrated heparin cofactor activity. They differed, however, in that the PEG-sup and AT III-HCF demonstrated considerably reduced progressive antithrombin function assessed over 30 min. This function was present in the PEG-ppt and S#2 fractions and this progressive activity was potentiated by EDTA. AT III two-dimensional immunoelectrophoresis (2-DIE) in the presence of heparin of both the Sephacryl and heparin-affinity purified components were very different, with the Sephacryl-purified AT III AGN showing both a fast peak and a very prominent slow-moving hump. The AT III heparin affinity fraction showed primarily a fast component. Dilution of the S#2 and PEG-ppt fractions resulted in considerable loss of the progressive activity and also the slow-moving component on 2-DIE. On the basis of these observations, it is postulated that AT III purified by PEG precipitation is in an aggregated form and that aggregate formation and dissolution is associated with AT III progressive activity.