THE concentration of Dorsal protein in the nucleus determines cell fate along the dorsoventral axis of the Drosophila embryo(1-13). The dorsal-group genes and the cactus gene are required for production and transmission of a localized signal an the ventral side of the embryo(4,5) which determines the position of the highest nuclear concentration of Dorsal protein(1-3) The ventralizing signal produced in somatic cells(6) is transmitted through the perivitelline space(7) to the integral membrane protein Toll(8). Inside the embryo it leads to dissociation of the cytoplasmic Dorsal-Cactus complex and subsequent nuclear localization of Dorsal protein(9,10). Two components are known to mediate the signal transduction between Toll and Dorsal-Cactus(11,12) : Pelle, a serine/threonine protein kinase(13), and Tube, a protein with an unknown biochemical activity(14). Here we construct gain-of-function alleles of pelle and tube and show that pelle functions downstream of tube. In addition, Pelle and Tube interact directly with one another. We propose that Tube is a direct activator of the protein kinase Pelle.