The aim of research is to develop potential tumor-targeted circulation-prolonged macromolecular magnetic resonance imaging (MRT) contrast agents without the use of low molecular gadolinium (Gd) ligands. The contrast agents were based on polymer metal complex nanoparticles with controllable particle size to achieve the active and passive tumor-targeted potential. In particular, poly (amidoamine) (PAMAM) dendrimer with 32 carboxylic groups was modified with folate-conjugated poly (ethyleneglycol) amine (FA-PEG-NH2, M-w: 2 k and 4 kDa). FA-PEG-PAMAM-Gd macromolecular MRI contrast agents were prepared by the complex reaction between the carboxylic groups in PAMAM and GdCl3. The structure of FA-PEG-PAMAM-COOH was confirmed by nuclear magnetic resonance (H-1-NMR), Fourier transform infrared (FTIR) spectra, and electrospray ionization mass spectra (ESI-MS). The mass percentage content of Gd (III) in FAPEG-PAMAM-Gd was measured by inductively coupled plasma-atomic emission spectrometer (ICP-AES). The sizes of these nanoparticles were about 70 nm measured by transmission electron microscopy, suggestion of their passive targeting potential to tumor tissue. In comparison with clinically available small molecular Gadopentetate dimeglumine, FA-PEG-PAMAM-Gd showed comparable cytotoxicity and higher relaxation rate, suggestion of their great potential as tumor-targeted nanosized macromolecular MRI contrast agents due to the overexpressed FA receptor in human tumor cell surfaces. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 1805-1814, 2010