In an olfactometer study on the response of the anthropophilic malaria mosquito Anopheles gambiae s.s. (Diptera, Culicidae) to human sweat it was found that freshly collected sweat, mostly of eccrine origin, was attractive, but that incubated sweat was significantly more attractive than fresh sweat. The behavioural response to l-lactic acid and ammonia, the main constituents of sweat, was investigated. l-lactic acid was attractive at one concentration only (11.11 mm) and removal of the l-lactic acid from the sweat by enzymatic decomposition did not affect the attractiveness of sweat. Ammonia caused attraction over a range of 0.113.4 m on glass slides and at 0.848.40 mol/min in an air stream. It is concluded that: human sweat contains kairomones for host-seeking An. gambiae; ammonia is an important kairomone for this mosquito; and that l-lactic acid is not a prerequisite in the attraction of An. gambiae to sweat.