Organisms known to cause bovine mastitis, Enterococcus faecalis ssp. liquefaciens ATCC 27959, Staphylococcus aureus ATCC 29740, Streptococcus agalactiae ATCC 27956, Streptococcus equinus ATCC 27960, Streptococcus dysgalactiae ATCC 27957, Streptococcus uberis ATCC 27958, and the neotype Staphylococcus epidermidis ATCC 14990 were examined for their susceptibility to die small peptide antibiotic, nisin. Using a disc assay, minimum inhibitory concentrations of nisin ranged from 10 to 250 μg/ml among the strains. Examination of die antimicrobial effect of 50 μg/ml nisin in milk showed nisin inhibited all gram-positive pathogens tested.