Eleven out of 24 premises sold alcohol to underage people during a police swoop in Derby city centre. Off-licences and bars were targeted in the initiative, which involved plain-clothed officers and 16-year-old volunteers visiting the premises.
Sgt Katie Hobbs said the results of the project were "very disappointing."
Staff who sold alcohol to underage drinkers were fined �80 or reported for summons. Police said further operations would be held in the near future.
The initiative was part of Operation Relentless - a two-month crackdown on crime in the county.