Epsilon(gamma-glutamyl)lysine isodipeptide has been detected in normal human plasma by a sensitive HPLC technique in a concentration of 1.9-3.6 mumol/l. Incubation of in vitro clotted plasma at 37 degrees C for 12 h resulted in an increased amount of isodipeptide, and there was no further significant change when streptokinase was also present. Increased in vivo isodipeptide concentrations were also observed in hypercoagulable states and during fibrinolytic therapy.