Little is known about gap junctional communication (GJC) in endothelial progenitor cells (EPC) which have the capacity to differentiate to mature endothelial cells (EC). GJC of progenitor cells with the mature endothelium may be necessary in vascular repair processes to achieve full functional competence. Therefore we analyzed the connexin (Cx) expression pattern of murine EPC isolated at day 7.5 (Hatzopoulos 1998) and measured GJC. Using RT-PCR and Western Blotting we found all three vascular connexins Cx37, Cx40 and Cx43 expressed in HUVEC, whereas EPC specifically expressed only Cx43. To further investigate if functional gap junctions were formed we used Calcein AM, a fluorescent dye which after intracellular hydrolysis can only be transferred from cell to cell via gap junctions. HUVECs and HeLa cells transfected with either Cx37, Cx40 or Cx43 were stained with calcein AM (200nM) and then co-incubated with membrane- labelled EPC. Following 6h co-incubation the fluorescence intensity was evaluated by FACS analysis. Up to 40% of the EC, 64% of the Cx43-HeLa and 60% of the Cx37-HeLa showed dye transfer from EPC. In contrast virtually no transfer was detected from Cx40-HeLa (5%) (n=2-4 each). We conclude that EPC express specifically Cx43 and are able to form functional gap junctions with EC. This coupling is mediated by Cx43/43 homo- and Cx43/Cx37 hetero-channels.