Saccharomyces strains of two types (K1+R1+ and K2+R2+) kill each other and K-R--sensitive strains by secreting protein toxins. K1 killer strains carry a 1.25 X 10(6) dalton double-stranded RNA plasmid, [KIL-k1], while K2 killers have a 1.0 X 10(6) dalton double-stranded RNA plasmid, [KIL-k2]. Mating [KIL-k1] haploids with [KIL-k2] haploids yields only [KIL-k1] diploids, that is, [KIL-k1] excludes [KIL-k2]. [EXL], a new non-Mendelian genetic element from a nonkiller strain, excludes [KIL-k2] but does not exclude [KIL-k1]. A second new non-Mendelian genetic element, called [NEX], when present prevents [EXL] from excluding [KIL-k2]. [NEX] does not prevent [KIL-k1] or [KIL-s1] (a suppressive mutant of [KIL-k1]) from excluding [KIL-k2]. A chromosomal gene, called MKT1, is needed for maintenance of [KIL-k2] if [NEX] is present. In the absence of [NEX], [KIL-k2] does not need MKT1. [KIL-k1] does not need MKT1 even if [NEX] is present. [EXL] replication depends on at least the products of MAK1, MAK3, MAK10 and PET18. [NEX] replication depends on MAK3 but is independent of MAK4, MAK6, MAK27 and MKT1.