SummaryK1 killer toxin is a secreted, pore‐forming protein that kills sensitive yeast cells. The heterodimeric toxin is processed from a precursor in the Golgi, and has allowed identification of the KEX2‐ and KEX1‐encoded proteases. The toxin binds to a glucan receptor on the cell wall of target yeast, and mutational analysis implicates both the a‐ and |5‐toxin sub‐units in receptor binding. Toxin‐resistant mutants with altered cell‐wall glucans have helped to outline a pathway of assembly of these polysaccharides. Patch‐clamp technology has demonstrated the nature of the lethal channel in toxin‐treated plasma membranes. The hydrophobic α‐subunit‐encoding region is the site of all mutations affecting channel formation. Immunity to the toxin is conferred by the toxin precursor, and immunity mutations map to the region encoding the α subunit. The precursor probably competes with the toxin to prevent channel formation in toxin‐producing cells, but the basis of this remains unknown. This toxin/immunity system has a domain structure that differs from that of other characterized toxins and has no known homologues.