The “killer plaque” technique ( 1 ) appears to be the simplest of all versions of the selection procedure ( 2 ), based on application of killer toxins, that have been examined in our laboratory ( see Chapter 33 ). The procedure is based on the observation that the selection of killer hybrids, cybrids, or transformants that produce killer toxin K1 or Kluyveromyces lactis killer toxin does not require application of killer toxin. The toxin production from the cells of a single colony is large enough to make a small zone (plaque) in the lawn of regenerated cells of sensitive parental strain in a minimal agar medium (Fig. 1 ). The parental auxotrophic killer strain can neither grow nor produce plaques under these conditions.   Fig. 1.  Demonstration of “killer plaques.” Petri dish and an enlarged section from it. Colonies of hybrid and cybrid clones (prototroph, K + , R + ) of S. cerevisiae GRF18/ZH and S. cerevisiae S6/1 obtained by induced protoplast fusion are forming plaques in the lawn of sensitive cells of S. cerevisiae S6/1.