We describe a three-protein signal-transduction pathway that governs immunity to a protein toxin involved in cannibalism by the spore-forming bacterium Bacillus subtilis. Cells of B. subtilis enter the pathway to sporulate under conditions of nutrient limitation but delay becoming committed to spore formation by killing nonsporulating siblings and feeding on the dead cells. Killing is mediated by the exported toxic protein SdpC. We report that extracellular SdpC induces the synthesis of an immunity protein, SdpI, that protects toxin-producing cells from being killed. SdpI, a polytopic membrane protein, is encoded by a two-gene operon under sporulation control that contains the gene for an autorepressor, SdpR. The autorepressor binds to and blocks the promoter for the operon. Evidence indicates that SdpI is also a signal-transduction protein that responds to the SdpC toxin by sequestering the SdpR autorepressor at the membrane. Sequestration relieves repression and stimulates synthesis of immunity protein.