Some blood-brain barrier properties of microvascular endothelial cells have been shown to be inducible by astrocytes. We tested the hypothesis that this cellular interaction is mediated by a soluble factor(s). Chick chorioallantoic vessels in ovo were constantly exposed to astrocyte-conditioned medium. We found that endothelial cells exposed to astrocyte-derived factors, but not to glioma- or endothelial cell-derived factors, expressed the HT7 antigen and neurothelin, two specific markers of the blood-brain barrier phenotype. These results indicate that a soluble factor(s) secreted by astrocytes is capable to induce specific blood-brain barrier properties in endothelial cells of non-neural origin.