The characteristic properties of the blood-brain barrier forming brain capillary endothelial cells (BCEC) are modulated by their microenvironment, but the cellular sources of the induction signals are still unclear. Apart from astrocytes, another cell type in close contact with cerebral blood vessels are the perivascular macrophages, which are known to be regularly replaced by blood-derived monocytic precursor cells. It is unknown if and how these cells may interact with the cerebral endothelium and modulate its BBB-specific functions. In the present study, a cell culture model of the BBB was used to investigate the effect of blood-derived human macrophages on the permeability of cultured bovine and human BCEC, determined by a transendothelial electrical resistance (TEER) measurement. We found that the TEER of postconfluent BCEC was considerably increased by a non-contact coculture with macrophages. After 24 h, we found a TEER augmentation of over 50% compared to the control without coculture, and this effect was comparable to the response of BCEC to a C6 glioma cells coculture. Stimulation or HIV-1 infection of the macrophages did not alter their effect on BCEC monolayer permeability. Investigation of signal transduction pathways showed that TEER increase of BCEC due to macrophages coculture was cAMP-independent and involves neither phospholipase C, protein kinase C nor calmodulin. Our findings demonstrate that macrophages are able to modulate BBB-specific functions in cultured BCEC. Thus, these cells or cerebral cells of monocytic origin (e.g. perivascular macrophages), may be part of the microenvironment of BCEC that modulates their specific properties in vivo.