Saliva composition in Aedes aegypti mosquitoes of differing ages was compared by acrylamide gel-electrophoresis. At o and 3 days after adult emergence the composition differed compared with 5 and 10-day old mosquitoes. Immediate type hypersensitivity reactions in a human volunteer were less pronounced towards 3 than 11-day old mosquitoes. Course and duration of the feeding process appeared not to be age-dependent.