BOS1 encodes an integral endoplasmic reticulum (ER) membrane protein and genetically interacts with three other yeast genes (BET1, SEC22, and YPT1) whose products are required for membrane traffic between the ER and the Golgi apparatus. Using an assay that reconstitutes transport at this stage of the pathway, we find that anti-Bos1p antibody blocks protein export after vesicles bud from the ER but prior to fusion with the Golgi. Additionally, the depletion of Bos1p from the ER leads to the formation of transport-incompetent vesicles. Carrier vesicles, immunoisolated with anti-Bos1p antibody, are approximately 50 nm in size. These vesicles contain Bos1p, Sec22p, and Ypt1p, but not Bet1p. The functional interactions of Bos1p with Ypt1p and Sec22p may be necessary for the fusion competence of the ER to Golgi transport vesicles.