We have previously shown that p115, a vesicle docking protein, binds to two proteins (p130 and p400) in detergent extracts of Golgi membranes. p130 was identified as GM130, a Golgi matrix protein, and was shown to act as a membrane receptor for p115. p400 has now been identified as giantin, a Golgi membrane protein with most of its mass projecting into the cytoplasm. Giantin is found on COPI vesicles and pretreatment with antibodies inhibits both the binding of p115 and the docking of these vesicles with Golgi membranes. In contrast, GM130 is depleted from COPI vesicles and inhibition of the GM130 on Golgi membranes, using either antibodies or an NH2-terminal GM130 peptide, inhibits p115 binding and vesicle docking. Together these results suggest that COPI vesicles are docked by giantin on the COPI vesicles and GM130 on Golgi membranes with p115 providing a bridge.