Abstract
1. The serum prepared by subjecting Cheddar cheese to pressure becomes inhibitory to the growth of lactobacilli as the cheese ripens.2. One factor responsible for the inhibitory phenomena is the increasing osmotic pressure of the serum due mainly to the increasing amounts of amino-acids present.3. Observations made during the ripening of a normal Cheddar cheese suggest that other factors, e.g. amino-acid imbalance, are also important.