We describe here an in vitro behavioral assay for testing mosquito repellents applied in a dose-based manner to a warm body (34°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 µg/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 µg/cm² (5 nmol/cm²) and the corresponding ED₉₅ at 4.12 µg/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.