The rel/NF-kappa B-related morphogen dorsal is a maternally expressed gene which is involved in the control of the dorso-ventral axis during early embryogenesis of Drosophila. We show that this gene is also expressed in the fat body of larvae and adults of Drosophila as well as in a tumorous blood cell line: its expression is noticeably enhanced upon bacterial (or lipopolysaccharide) challenge. This challenge also induces within 15-30 min a nuclear translocation of the dorsal protein. The genes encoding inducible antibacterial peptides in Drosophila contain kappa B-related nucleotide sequences and we show that the dorsal protein can bind to such motifs and sequence-specifically transactivate a reporter gene in co-transfection experiments with a Drosophila cell line. However, in dl1 mutants, in the absence of dorsal protein, the genes encoding antibacterial peptides retain their inducibility, suggesting a multifactorial control. The results indicate that in addition to its role in embryogenesis, dorsal is involved in the immune response of Drosophila. They also strengthen the analogy between the mammalian acute phase response and the insect immune response.