Insects possess a powerful immune system, which in response to infection leads to a vast production of different antimicrobial peptides. The regulatory regions of many immunity genes contain a GATA motif in proximity to a kappaB motif. Upon infection, Rel proteins enter the nucleus and activate transcription of the immunity genes. High levels of Rel protein-mediated Cecropin A1 expression previously have been shown to require the GATA site along with the kappaB site. We provide evidence demonstrating that the GATA motif is needed for expression of the Cecropin A1 gene in larval fat body, but is dispensable in adult fat body. A nuclear DNA-binding activity interacts with the Cecropin A1 GATA motif with the same properties as the Drosophila GATA factor Serpent. The GATA-binding activity is recognized by Serpent-specific antibodies, demonstrating their identity. We show that Serpent is nuclear in larval fat body cells and haemocytes both before and after infection. After overexpression, Serpent increases Cecropin A1 transcription in a GATA-dependent manner. We propose that Serpent plays a key role in tissue-specific expression of immunity genes, by priming them for inducible activation by Rel proteins in response to infection.