Genes coding for antimicrobial peptides in amphibia reveal a remarkably high number of structural motifs for response elements, previously identified in the genes of insect antimicrobial peptides and in those of the mammalian acute phase response. This study focuses on the functional analysis of the bombinin gene promoter in a Drosophila blood cell line, and the identification of kappaB-binding factors in skin secretions of the frog Bombina orientalis. Transfection experiments demonstrated that the bombinin gene promoter was activated in a lipopolysaccharide-dependent manner, and that insect Rel factors target specific sequences in the amphibian gene promoter. After bathing frogs in bacteria, their skin secretions contained kappaB-specific binding complexes, indicating that Rel factors are crucial components in the response against gram-negative bacteria in this species. These results suggest that a common ancestral control mechanism governs the expression of the first line host-defence from insects to vertebrates.