We describe here an in vitro behavioral assay for testing mosquito repellents applied in a dose-based manner to a warm body (34 degrees C) in test cages. The system was used to assess the sensitivity of 4-6-day-old Anopheles gambiae to the insect repellent diethyl methyl benzamide (deet). These tests were made in the absence and presence of additional carbon dioxide (CO2) applied as a pulse to activate mosquitoes in the cages. In the absence of the CO2 pulse the mosquitoes hardly responded to the warm body. Increasing the CO2 level in the cage by 1,000 parts per million caused a 25-fold increase in the number of landings by mosquitoes on the warm body in 2-min tests. This mosquito activation allowed the measurement of a significant reduction in the number of landings to bite on the warm body with increasing doses of deet (0.4 to 3.8 microg/cm2). An asymptotic nonlinear model fitted to the repellency data in the presence of CO2 allowed estimation of the effective dose of deet that reduced landings to bite by 50% (ED50) at 0.95 microg/cm2 (5 nmol/cm2) and the corresponding ED95 at 4.12 microg/cm2 (21.5 nmol/cm2). This in vitro bioassay has the advantage of permitting a fast throughput of test products under standardized conditions and is suitable for screenings designed for the purpose of discovering lead products with as yet unknown human toxicological and dermatological profiles.