The Escherichia coli beta-glucuronidase gene has been used as a marker gene to monitor a killer Saccharomyces cerevisiae strain in mixed-culture ferments. The marked killer strain was cured of its M-dsRNA genome to enable direct assessment of the efficiency of killer toxin under fermentation conditions. Killer activity was clearly evident in fermenting Rhine Riesling grape juice of pH 3.1 at 18 degrees C, but the extent of killing depended on the proportion of killer to sensitive cells at the time of inoculation. Killer activity was detected only when the ratio of killer to sensitive cells exceeded 1:2. At the highest ratio of killer to sensitive cells tested (2:1), complete elimination of sensitive cells was not achieved.