Nisin in brain heart infusion medium (Difco) and Bacto-Staphylococcus medium 110 completely inhibited the growth of Staphylococcus aureus 196. This restraining action of nisin on S. aureus was not as effective in the presence of whole milk. This was especially noted when attempts were made to control growth of S. aureus in cheese made with starter inactivated by phage. Although nisin had some inhibitory action on S. aureus in cheese it was not equal to that exerted by an active Streptococcus lactis culture. Its presence failed to prevent production of enterotoxin A. A factor influencing the failure of nisin to control growth of S. aureus in cheese was the presence of butterfat which was shown to interfere with the action of nisin against the growth of S. aureus.