Aims: To investigate the occurrence and extent of Saccharomyces cerevisiae and Oenococcus oeni interactions. Methods and Results: Interactions between S. cerevisiae and O. oeni were investigated by double- layer and well-plate assays showing the occurrence of specific interactions for each yeast - malolactic bacteria ( MLB) coupling. Heat and protease treatments of synthetic grape juice fermented by the S. cerevisiae strain F63 indicated that the inhibitory activity exerted by this yeast on O. oeni is due to a proteinaceous factor( s) which exerts either bacteriostatic or bactericidal effect depending on concentration and affects malolactic fermentation in natural grape juice and wine. Conclusions: A proteinaceous factor( s) produced by a S. cerevisiae wine strain able to inhibit O. oeni growth and malic acid fermentation was characterized. Significance and Impact of the Study: The individuation, characterization and exploitation of yeast proteinaceous factor( s) exerting inhibitory activity on MLB may offer new opportunities for the management of malolactic fermentation.