In polarized epithelial cells, influenza virus buds exclusively from the apical domain of the plasma membrane, whereas vesicular stomatitis virus (VSV) buds exclusively from the basolateral domain. In virus-infected cells, the envelope proteins, influenza hemagglutinin (HA) and vesicular stomatitis virus G (VSV G), are likewise transported to and localized in the same domain of the plasma membrane from which the viruses bud. Previous studies have shown that influenza HA and VSV G proteins, when expressed from cloned cDNAs, are accumulated preferentially on the proper domains (apical and basolateral, respectively), indicating that the signal(s) for polarized transport resides in the polypeptide backbone of the proteins. To further elucidate the structural features required for apical vs. basolateral transport, we have constructed a gene that encodes a chimeric protein (H1GA) containing the external domain of HA and the transmembrane and cytoplasmic domains of VSV G. When the chimeric protein (H1GA) is expressed in CV1 cells using a simian virus 40 late expression vector, it is transported to the cell surface with kinetics similar to that of the native HA protein. Further, the chimeric protein, when expressed in polarized MDCK cells using a vaccinia virus early expression vector, is transported only to the apical surface, suggesting that the ectodomain of HA contains a signal for apical transport.