SUMMARY Killer strains of yeast, Saccharomyces cerevisiae, liberate a killer factor into the medium which kills senstive yeast strains. The growth conditions necessary for the production of stable high-titre killer solutions and a biological assay for the killer factor are described. Purification was achieved by fractional pre- cipitation with (NH&SO,, dialysis, gel filtration and ultrafiltration. The fractionated killer factor is an unstable macromolecular protein which is inactivated by papain. The death of sensitive cells is not coincident with absorption of the killer factor, but can be delayed or prevented by variations in the environmental conditions. Sensitive cells are most susceptible to the action of the killer factor when in log phase. Treated resting cells on entering log phase are killed immediately.