No other exogenous protein enters the central nervous system from the circulation as readily as tetanus toxin. We examined the capability of the non-toxic binding fragment of tetanus toxin (C-Fragment) so serve as a vehicle for transport of other proteins into the mouse CNS. Using periodate oxidation of the enzyme horseradish peroxidase (HRP), we synthesized two separate macromolecular complexes, one containing C-fragment and HRP, and the other C-fragment, HRP and a third "test" protein-human IgG. The distribution of C-fragment-HRP was typical of blood borne proteins including native C-fragment, with labeling of all neurons with known projections outside the blood-brain barrier, particularly large spinal motoneurons. C-fragment-HRP conjugates showed superior neuronal labeling to over 100-fold greater quantities of free HRP. Complexes containing C-fragment, HRP and human IgG were internalized by neurons from both intramuscular and intraperitoneal injections. The efficiency of neuronal uptake of IgG in the C-fragment conjugated form was enhanced over 40-fold compared to free IgG. Linkage of a large protein to C-fragment probably leads to enhanced endocytosis of that protein by neuronal terminals projecting outside the blood-brain barrier. C-fragment can serve as a vehicle to allow selected proteins to bypass the barrier and enter the CNS.