The paper investigates effects of pectolytic enzyme treatments of red grape mashes on the microbiological quality of wines and wine stability with counting of the presence yeast cells (Saccharomyces cerevisiae), after incubation on Sabourald-maltose agar. Also wine samples were investigated for the presence of moulds and other bacteria, including dangerous bacteria Salmonella and Shigella, Staphylococus aureus, Proteus spp., Sulphite-reducing clostridy and Escherichia coli. All wine samples the results showed the presence of yeasts Saccharomyces cerevisiae. Saccharomyces is regarded as spoilage organism only if it is found in the wrong place at the wrong time (e.g. in a bottle of semi-sweet wine) causing re-fermentation. Pectinolytic enzyme preparation Trenolin Rot DF showed the best results. Pectolytic enzyme treatments of red grape mashes had a pronounced effect on the microbiological quality of wines and wine stability. Results from comparison of effects of pectolytic enzyme preparations in winemaking on the microbiological quality of wines can contribute to a better orientation in the choice of suitable enzyme preparations in wine industry. UDC: 579.66:579.67+663.2 Key words: Pectolytic enzymes, red grape Merlot, microbiological quality of wines, microbial spoilage in wine.