The presence of transport systems at the blood-brain barrier (BBB) enables some cytokines in blood to reach specific targets in the brain and spinal cord. The "transporters" function in a way different from conventional receptors, in that cytokines are chaperoned from blood to the CNS rather than being degraded in the specialized endothelial cells composing the BBB. Here we present the first study to determine whether the transporter for tumor necrosis factor-alpha (TNFalpha) is identical to its receptors. Three types of TNFalpha receptor knockout mice were used, and the influx of (125)I-TNFalpha from blood to brain and blood to spinal cord was measured. In either p55 or p75 receptor knockout mice, the influx of (125)I-TNFalpha was significantly, but not completely, decreased in spinal cord, whereas the decrease in brain was not statistically significant. This indicates that both receptors are partially involved in the transport of TNFalpha across the BBB but that neither receptor is the sole transporter. By contrast, in double knockout mice lacking both p55 and p75 receptors, the entry of (125)I-TNFalpha into brain and spinal cord was completely abolished. Therefore, both receptors are necessary for transporting TNFalpha across the BBB. The results clearly demonstrate that the transport of TNFalpha across the BBB is a complicated process involving additive or even synergistic activities of both receptors, thus differing from typical ligand-receptor binding and downstream signal transduction.