An enzyme-linked immunosorbent assay (ELISA) was used to measure the quantity of Rift Valley fever (RVF) viral antigen in infected Egyptian Culex pipiens. Infectivity, as measured by plaque assay, was significantly correlated with viral antigen, as measured by the ELISA, in all groups of mosquitoes regardless of the time interval after the infectious blood meal. The proportion of noninfectious viral antigen in these groups increased with time. When individual mosquitoes were assayed the plaque assay and the ELISA techniques had similar sensitivity (100% vs. 93%, respectively) and specificity (94% vs. 94%, respectively) in detecting mosquitoes capable of transmitting virus to susceptible hamsters. The ELISA may be useful in detecting RVF-infected arthropods in the field because it provides a rapid, sensitive, and specific test.