The yeast Zygosaccharomyces builii strain 412 was found to liberate a killer toxin (KT412) lethal to sensitive strains of Saccharomyces cerevisiae and Candidu glubrutu. Culture supernatants of the killer strain were concentrated by ultrafiltration and the extracellular protein was purified by gel filtration and ion-exchange chromatography. Gel filtration and SDS-PAGE of the electrophoretically homogeneous killer protein indicated an apparent molecular mass of 10 kDa. The killer toxin KT412 is probably not glycosylated since it did not show any detectable carbohydrate structures. KT412 was bound to sensitive but not to resistant yeast cells. The mannan, and not the glucan, fraction of the cell wall of the sensitive yeast was the primary target for the killer toxin binding. The killer strain 2. bailii 412 contained three double-stranded RNA plasmids of 1.9, 2.9 and 4.0 kb. Curing by cycloheximide resulted in the concomitant loss of killer activity and the 1.9 kb dsRNA species that is therefore regarded as equivalent to the killer-toxin-coding M-plasmids of S. cerevisiae.