1. Cultures ofStr. faecalis, L. plantarumand a strain ofLeuconostoc, when added to cheese milk, all brought about an increase in Cheddar flavour intensity in the resultant cheese.2. Examination of the cheeses for tyramine content showed that the formation of the amine bore no relationship to the increases in flavour intensity. The production of tyramine appeared to be merely incidental in the cheese-ripening process.3. A survey among the lactic acid bacteria disclosed only one species containing a significant amount of the L( – )-tyrosine decarboxylase enzyme—L. brevis. This species added to cheese milk causes an objectionable ‘yeasty’ flavour in the final cheese.