The membrane anchor for the molecular motor kinesin is a critical site involved in intracellular membrane trafficking. Monoclonal antibodies specific for the; cytoplasmic surface of chick brain microsomes were used to define proteins involved in microtubule-dependent transport. One of four antibodies tested inhibited plus-end-directed vesicle motility by approximately 90 percent even as a monovalent Fab fragment and reduced kinesin binding to vesicles. This antibody bound to the cytoplasmic domain of kinectin, an integral membrane protein of the endoplasmic reticulum that binds to kinesin. Thus, kinectin acted as a membrane anchor protein for kinesin-driven vesicle motility.