Seventy-five Lactococcus lactis strains were screened for their inhibitory effect on Staphylococcus aureus growth in milk. Most lactococcal strains had a strong antagonistic effect. Characterization of this effect showed that acidification was not involved in the inhibition observed within the first 24 h of mixed culture. Alternate effects such as bacteriocin- or hydrogen peroxide-production were eliminated. These results question some generally accepted ideas and show that even low acidifying L. lactis strains, widely used in raw milk soft cheeses, can efficiently inhibit S. aureus growth even with initial contamination levels as high as 103 cfumL