In a new bioassay a small Y-shaped wind tunnel is used to quantitatively investigate the responses of mosquitoes to host odours. Female yellow fever mosquitoes, Aedes aegypti (L.) (Diptera: Culicidae) were tested to (1) a human hand, (2) an extract with human skin residues, (3) L-(+)-lactic acid, and (4) carbon dioxide. The responses to the skin extract followed a sigmoid dose response curve. The most effective dose attracted 80–90% of the mosquitoes within 30 s and was as effective as the human hand. L-(+)-lactic acid was identified in this extract and found attractive for mosquitoes also when presented alone. Carbon dioxide stimulated taking flight and was attractive, an effect which was synergistically enhanced in combination with L-(+)-lactic acid. The presented bioassay is especially suited to test the behavioural effects of synthetic odours as well as of natural odour sources. Due to the fast response of the mosquitoes, the sensitivity, and the simplicity of the testing procedure it is a potent tool in the search for new attractive components.