The K1 killer plasmid, [KIL-k1], of Saccharomyces cerevisiae is a 1.25 x 106 dalton linear double-stranded RNA plasmid coding for a protein toxin and immunity to that toxin. The [KIL-sd1] plasmid is a replication-defective mutant of [KIL-k1] that depends on one of the recessive chromosomal superkiller (ski-) mutations for its maintenance (Toh-e and Wickner 1979). This report concerns a means by which [KIL-sd1] can be stably maintained in a SKI+ host. Strains carrying a plasmid we call [HOK] (helper of killer) stably maintain [KIL-sd1]. [HOK] segregates 4 [HOK]:0 in meiotic crosses and is efficiently transferred by cytoplasmic mixing (heterokaryon formation). [HOK] depends for its maintenance on the products of PET18, MAK3, and MAK10, three chromosomal genes needed to maintain [KIL-k1], but is independent of 10 other MAK genes and of MKT1. [HOK] is not mitochondrial DNA and is unaffected by agents which convert ψ+ strains to ψ-. [HOK] is also distinct from the previously described plasmids [URE3], 20S RNA, 2 µ DNA, and [EXL]. Strains lacking [HOK] consistently have a four-fold lower copy number of L double-stranded RNA than strains carrying [HOK].