Dominant suppressors of the first type nonsense mutations, ade1-6, hisX, lys9-A21 and leu2-2, inhibit the killer activity in Peterhoff genetic stocks of Saccharomyces cerevisiae. Phenotypes CN (weak killer at 20 degrees, neutral at 32 degrees) and N (neutral at both temperatures) arising in the presence of the suppressors in killer strains are dominant toward S phenotype and toward K phenotype.