Recently, we have shown that transferrin (Tf) is actively endocytosed by the Tf R on primary cultured bovine brain capillary endothelial cells (BCEC). The objective of this investigation is to determine whether the Tf R can facilitate endocytosis of a (protein) model drug, using Tf as a targeting vector. Secondly, the mechanism of endocytosis was investigated. Horseradish peroxidase (HRP, 40 kDa) was chosen as a model drug, since it normally does not cross the blood-brain barrier (BBB) and its concentration in biological media can be easily quantified. Tf-HRP conjugates (1:1) are actively and specifically endocytosed by BCEC in vitro in a concentration and time-dependent manner. At an applied concentration of 3 microg/ml, association (a combination of binding and endocytosis) of Tf-HRP reached equilibrium at a concentration of 2 ng/mg cell protein after 1 h of incubation at 37 degree C. This was approximately 3-fold higher compared to binding at 4 degree C (0.6 ng/mg cell protein). Association of Tf-HRP was compared to BSA-HRP. After 2 h of incubation at 37 degree C association levels were 5.2 and 2.5 ng/mg cell protein, for Tf-HRP and BSA-HRP, respectively. Under those conditions, association of Tf-HRP could be inhibited to approximately 30% of total association by an excess of non-conjugated Tf, but not with BSA, while association of BSA-HRP could be inhibited by both proteins. Furthermore, by using specific inhibitors of endocytotic processes, it was shown that association of Tf-HRP is via clathrin-coated vesicles. Association of Tf-HRP is inhibited by phenylarsine oxide (an inhibitor of clathrin-mediated endocytosis) to 0.4 ng/mg cell protein, but not by indomethacin, which inhibits formation of caveolae. Finally, following iron scavenging by deferoxamine mesylate (DFO, resulting in a higher Tf R expression) a 5-fold increase in association of Tf-HRP to 15.8 ng/mg cell protein was observed. In conclusion, the Tf R is potentially suitable for targeting of a (protein) cargo to the BBB and to facilitate its endocytosis by the BCEC.