The major form of cross-link found in apolipoprotein B was identified as N1N12-bis-(gamma-glutamyl)spermine, a product known to be formed through the catalytic action of transglutaminases (EC 2.3.2.13). N1-(gamma-Glutamyl)spermine was present in a trace amount but epsilon-(gamma-glutamyl)lysine cross-links, which are formed during fibrin formation in plasma, were not detected. In the presence of catalytic amounts of plasma Factor XIIIa (a thrombin-dependent extracellular transglutaminase) or cellular transglutaminase (a cytosolic enzyme), apolipoprotein B and other plasma apolipoproteins (A-I, A-II and C) underwent covalently bridged polymerization and served as amine acceptor substrates. These results suggests that transglutaminases may participate in the covalent modification of apolipoproteins, either in the physiological state or during pathogenesis.