Adenocarcinomas in rats and humans frequently contain perivascular, degranulating mast cells that release heparin. Protamine is a low-molecular weight, cationic polypeptide that binds to heparin and neutralizes its anticoagulant properties. A novel magnetic resonance imaging (MRI) contrast agent containing protamine was synthesized. TTDASQ, the derivative of TTDA (3,6,10-tri(carboxymethyl)-3,6,10-triazadodecanedioic acid), was also synthesized and the kinetic stability of [Gd(TTDASQ)]- chelate containing phosphate buffer and ZnCl2 to measure the relaxation rate (R1) at 20 MHz was studied by transmetallation with Zn(ii). The water-exchange rate (kex298) of [Gd(TTDASQ)]- is 6.4 × 106 s-1 at 25.0 ± 0.1 °C which was obtained from the reduced 17O relaxation rates (1/T1r and 1/T2r) and chemical shift (ωr) of H 217O, and it is compared with that previously reported for the other gadolinium(iii) complex, [Gd(DO3ASQ)]. The binding affinity assay showed that the (TTDASQ)3-pro19 has higher activity toward heparin. On the other hand, the effect of heparin on the relaxivity of the [Gd(TTDASQ)3-pro19] conjugate shows the binding strength (KA) is 7669 dm3 mol-1 at pH 7.4 and the relaxivity (rb1) of the [Gd(TTDASQ)3-pro 19]-heparin adduct is 30.9 dm3 mmol-1 s -1.