Various Sarcophaga peregrina (flesh fly) defense protein genes were shown to be activated when NIH-Sape-4 cells were cultured with bacterial lipopolysaccharides or beta-1,3-glucan. The 5' upstream regions of the defense protein genes were found to have common motifs showing similarity to the mammalian NF-kappa B-binding consensus sequence. A protein with affinity to the NF-kappa B-binding motif of the Sarcophaga lectin promoter was identified and purified to near homogeneity. This 59-kDa protein also bound to the NF-kappa B-binding motifs of other defense protein genes, e.g., sarcotoxin I and sarcotoxin II genes. This protein was found in both the cytoplasmic and the nuclear fractions of the cells, and it appeared to migrate from the cytoplasm to the nucleus on treatment of the cells with lipopolysaccharides. This 59-kDa protein is probably a transcriptional regulator of the genes for defense proteins of S. peregrina.