Among the most specific markers of the blood-brain barrier phenotype of endothelial cells are the well-characterized immunoglobulin-like surface glycoprotein HT7 and the probably related or identical glycoprotein neurothelin. Both can be induced in chorioallantoic vessels by transplants of embryonic mouse brain. Other blood-brain barrier markers have been shown to be inducible by type-1 astrocytes in endothelial cells of non-neural origin. In the present work we tested the hypothesis that this cellular interaction between astrocytes and endothelial cells is mediated by a soluble factor(s). Chorioallantoic vessels of embryonic day 9 chick embryos were exposed for 4 or 10 days to a constant and localized delivery of astrocyte-conditioned medium by using a piece of gelfoam posed onto the chorioallantoic membrane, as a localized reservoir, which was connected to a miniosmotic pump system delivering astrocyte-conditioned medium at a steady rate. We found that in a significant number of chorioallantoic vessels located near the gelfoam, endothelial cells exposed to astrocyte-conditioned medium for a period of 4 or 10 days, but not to glioma-, fibroblast- or endothelial cell-derived conditioned medium, expressed the HT7 antigen and neurothelin. These results provide evidence that type-1-astrocytes are capable of inducing blood-brain barrier related properties in endothelial cells of non-neural origin through a soluble factor(s)