Abstract: A phage-displayed library of llama single heavy domain antibodies (sdAbs) was enriched for species that selectively bind to and are internalized by human cerebromicrovascular endothelial cells (HCEC). From the enriched library, two sdAbs were selected, sequenced, subcloned, and expressed as fusion proteins with c-myc-His5 tags. Similarly as phage-displayed sdAbs, these soluble tagged sdAbs were shown to selectively bind to HCEC and to transmigrate across in vitro human blood-brain barrier (BBB) model. In contrast to an unrelated llama sdAb, these sdAbs were also detected in the brain after i.v. injection into mice. These small (˜13 kDa) antibody fragments have essential characteristics of brain-specific delivery vectors and can be used to facilitate drug transport across the BBB.