The highest activities of alcohol dehydrogenase were obtained when flor yeasts were grown on L-lactic acid as the main carbon source. The strains with the lowest average of alcohol dehydrogenase activities, grown on glucose and ethanol, make up the velum on wines during the early stages ageing. One of the strains studied (Saccharomyces cerevisiae, M10) may be a suitable source from which to isolate this enzyme (32 units of activity per mg protein).