a,-Antichymotrypsin purified from normal human serum was separated by affinity chromatography into three microheterogeneous forms on a concanavalin-A -Sepharose column: a pass-through (peak I), a retarded (peak 2) and a bound form (peaks 3 + 4). For each form the asparagine-linked carbohydrate chains were liberated as oligosaccharides by hydrazinolysis, submitted to reduction with NaBH4 after re-N-acetylation and further separated by affinity chromatography on a concanavalin-A -Sepharose column. The complete primary structure of the glycans was determined by high-resolution 'H-NMR spectroscopy. The results indicated the presence of disialyl diantennary and of trisialyl triantennary type glycanic structures, the latter being accompanied by traces of disialylated triantennary oligosaccharide. The N-glycanase was used for the deglycosylation of the unfractionated ccl-antichymotrypsin; the successive removal of the N-linked complex-type oligosaccharide side chains of u,-antichymotrypsin was studied in the presence of detergents. From these experiments it is concluded that alantichymotrypsin carries four oligosaccharide side chains. Moreover our results show that the peak l contains four triantennary glycans, the peak 2 three triantennary and one diantennary glycans while the bound peaks 3 + 4 possess, on average, about one triantennary and three diantennary glycans per molecule. Since we showed that the peak 4 contains mostly diantennary glycans, it can be deduced that in peak 3 there are molecules carrying two triantennary and two diantennary glycans and others carrying one triantennary and three diantennary glycans.Human a,-antichymotrypsin is a plasma glycoprotein with a relative molecular mass ( M J of approximately 58000 and a carbohydrate content of 24% [l]. It belongs to the superfamily of serpins (serine proteinase inhibitors) [2]. In addition to Creactive protein and serum amyloid A protein, cr,-antichymotrypsin is one of the major acute-phase proteins [3] since its serum concentration increases more than fourfold within a few hours in response to an inflammatory stimulus [4].The precise biological function of a,-antichymotrypsin has not been yet determined. It is known to inhibit chymotrypsinlike proteases [5 -71 by formation of equimolar complexes which, however, dissociate with time. Eriksson et al.[8] recently described four families with inherited partial al-antichymotrypsin deficiency in which heterozygotes have 50% of normal plasma levels. No homozygotes have yet been identified. a,-Antichymotrypsin has been shown to be identical to the protein termed 64DP [9] and has a high affinity for DNAcellulose.It has been claimed that a,-antichymotrypsin is a component of the brain amyloid deposits in Alzheimer's disease [lo] but we recently showed 1111 that no distinction can be made between extracts of 14 control brains and extracts of 12 Alzheimer brains with regard to the amount of al-antichymotrypsin.The complete amino acid sequence of al-antichymotrypsin deduced from the nucleotide sequence [I21 reveals ...