Raw bulk milk samples were stored at 5°C in the laboratory mostly for 7 or 8d and examined at intervals for bacterial content, proteolysis as indicated by tyrosine value, and organoleptic quality. Data are presented for these samples illustrating relationships between bacterial population counts, tyrosine values and organoleptic quality ratings. Bacterial counts (total bacteria, psychrotrophs and proteolytic psychrotrophs) generally exceeded 106/ml before a definite increase in tyrosine value or any deterioration in organoleptic quality could be detected. Tyrosine value was indicated to be useful as a general index of bacteriological and organoleptic quality of commercial raw milk subjected to prolonged cold storage. The tyrosine value procedure was also shown to have application for monitoring the development of proteolysis in cold-stored pasteurized milk and cream.