Species of 7 of the 28 yeast genera in the National Collection of Yeast Cultures exhibited killing activity against Saccharomyces cerevisiae. The highest incidence of killer yeasts was found in the genus Hansenula (12 of the 29 strains examined). Saccharomyces, the best represented genus in the Collection, showed a low incidence of killer activity and many of the killer strains are hybrids with a common S. cerevisiae parent. The activities of culture filtrates of the 59 killer yeast isolated responded differently to pH and four types of response were recognised.

In batch culture in a complex medium the killer yeasts NCYC 738 and NCYC 235 gave maximal killer activity when grown in the pH ranges 4·2–4·4 and 4·6–4·8 respectively. Incubation of culture filtrates of NCYC 738 for 10 h at 25 °C or 2 h at 29 °C resulted in a 50% reduction in activity. The addition of bovine serum albumin or gelatine to a complex medium stabilized killer activity. In a defined medium the addition of yeast extract stimulated the production of killer activity. When killer yeast NCYC 738 was grown in a chemostat, killer activity was influenced by temperature, pH and the rate at which the culture was stirred. The production of killer activity was growth‐linked and increased as dilution rate was raised to a maximum of 0·15 h”1. Steady state continuous cultures of the sensitive strain, NCYC 1006, were contaminated deliberately with either killer or killer‐cured strains. During the first 30 h cultivation, the cell concentration of both strains increased. Subsequently the sensitive strain was displaced from the culture. When killer‐cured NCYC 738 was added, the rate of displacement was proportional to the culture temperature. However, with killer NCYC 738 increase of temperature reduced the rate of displacement. When killer NCYC 235 was employed, a lowering of pH decreased the rate of displacement but had no effect when killer‐cured NCYC 235 was used.

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