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Using a new, sensitive assay of bacterial growth inhibition, inducible antibacterial activity has been identified in the haemolymph of the mosquito, Aedes aegypti following inoculation with bacteria or with microfilarie of the filarial nematode Brugia pahangi, but not after inoculation with sterile culture medium. A lower level of antibacterial activity has also been observed in untreated individual mosquitoes. Following bacterial inoculation, a basic, inducible antibacterial peptide has been detected using native PAGE at pH 4, which corresponds with a 4.5 kDa peptide detected by tricine SDS-PAGE followed by silver staining. A peptide has been purified from immune haemolymph by ultrafiltration, followed by reversed-phase HPLC, yielding a single major peak with antibacterial activity. Partial amino acid sequence analysis of this fraction has revealed substantial homology with insect defensins. The data are consistent with …