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Microinjection of antibodies against a synthetic peptide of a non-clathrin-coated vesicle-associated coat protein, β-COP, blocks transport of a temperature-sensitive vesicular stomatitis virus glycoprotein (ts-O45-G) to the cell surface. Transport is inhibited upon release of the viral glycoprotein from temperature blocks at 39.5°C (endoplasmic reticulum [ER]) and 15°C (intermediate compartment), but not at 20°C (trans-Golgi network). Ts-O45-G is arrested in tubular membrane structures containing p53 at the interface of the ER and the Golgi stack. This is consistent with inhibition of acquisition of endoglycosidase H resistance of ts-O45-G in injected cells. Secretion of endogenous proteins and maturation of cathepsin D are also inhibited. These data provide in vivo evidence that β-COP has an important function in biosynthetic membrane traffic in mammalian cells.