Human transferrin was covalently coupled to ultrasmall superparamagnetic iron oxide (USPIO) particles, and the transferrin-USPIO obtained was investigated in vivo in experimental SMT/2A tumor-bearing rats (rat mammary carcinoma). Physicochemical characterization showed an overall size of 36 nm (DLS) with a core size of 5 nm (TEM). Relaxivities were R1 = 23.6 and R2 = 52.1 liter/mmol.s (0.47 T). Bound transferrin was 280 micrograms/mg of iron. Pharmacokinetic investigations revealed a half-life of 17 min in normal rats. The MR evaluation of tumor signal intensity over time showed a 40% (range 25-55%) signal reduction 150 min after injection with the reduction persisting for at least 8 h. Control experiments using the parent USPIO compound or USPIO labeled with a nonspecific human serum albumin (HSA-USPIO) showed a change of only 10% (range 5-15%) in tumor signal intensity over time. The results demonstrate that a combination of the USPIO relaxivity properties with the specificity of transferrin-mediated endocytosis allows in vivo detection of tumors by MR imaging.