Serum collected 60 min after feeding ovalbumin (OVA) to BALB/c mice transfers specific tolerance to naive recipients via an unknown mechanism. We have now identified a large fragment of the OVA molecule as the putative active moiety. Specific absorption of immunoreactive OVA from tolerogenic serum by immunoaffinity chromatography removed the tolerogenic activity; following elution of the bound OVA and subsequent injection into naive recipients it was possible to demonstrate tolerogenic activity equivalent to that seen with unmanipulated serum. Passage of OVA tolerogenic serum through a bovine serum albumin (BSA)-specific affinity column had no effect on in vivo OVA tolerogenic activity. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting using anti-OVA antibodies demonstrated the presence of two bands with apparent molecular weights (MW) of 21,000 and 24,000 in the OVA-related material absorbed from the tolerogenic serum. The 24,000 MW moiety was also visualized by silver staining. These moieties are putative candidate tolerogens as they were absent from normal BALB/c mouse serum analysed 5 min after feeding OVA and from severe combined immunodeficiency (SCID) mouse serum--without tolerising activity--analysed 60 min after a similar feed.