In order to determine the requirements for secretion of flavivirus structural proteins, we analyzed the expression of several West Nile flavivirus gene cassettes of different lengths in vaccinia virus expression systems. Expression of the longest cassette coding for the 5'-nontranslated region, proteins C through NS2B, and the protease domain of NS3, resulted in secretion of prM-E complexes and cleavage of prM. The presence and proper processing of the NS2A-NS2B-NS3 region appeared to be necessary for prM-E secretion. These proteins were released from cells mostly as membranous complexes which may represent empty viral envelopes. Cleavage of the membrane-associated intracellular form of protein C (C(i)) to produce the virion form (Ce) appeared to be critical for release of viral proteins. The presence and proper cleavage of the NS2A-NS2B-NS3 region were also found to be necessary for efficient C-prM cleavage by signalases. The NS2B-NS3 complex was implicated in cleavage of the intracellular form of protein C. Formation of a low level of virus-like particles was detected by electron microscopy. A model for virion formation, suggesting a critical role of the NS2B and NS3 proteins, is discussed.