The use of enzyme immunoassay to detect St. Louis encephalitis (SLE) viral antigen in vector mosquitoes enhances the effectiveness of surveillance because infected mosquitoes can be identified more rapidly than with conventional virus isolation systems and because it is a simple and accessible procedure. Infectivity among mosquitoes experimentally infected with SLE virus was lost within 24 h after the mosquitoes were stored at 27 degrees C and 80% relative humidity; however, viral antigen remained stable under these conditions and could be detected by enzyme immunoassay 2 weeks later. Desiccation further extended the period during which antigen could be detected to 6 weeks. Absorbances were higher in infected mosquitoes stored at 27 degrees C than in mosquitoes frozen continuously. Absorbances in infected mosquitoes also increased after repeated freezing and thawing and sonication. Both phenomena may be related to the release of antigen from decaying or disrupted cells. The relative stability of SLE viral antigen at ambient temperatures lends flexibility to schemes which use direct antigen detection to identify vectors. Surveillance systems can be designed without regard to collecting living mosquitoes, and a cold chain in unnecessary to preserve specimens, thus reducing the cost of surveillance and expanding the geographic areas to which it is accessible.