Derbyshire police have been checking which shops and bars have sold alcohol to people under 18.
With the help of underage volunteers, officers visited 74 premises in Derby and South Derbyshire on 14 June.
Of the bars and pubs they visited, 25% sold alcohol to the teenagers, while 47% of the off-licences sold it to underage customers.
Officers said licence holders must be more careful and parents should take more responsibility.
Police and trading standards officers are working with staff on the premises which failed their checks to stop it happening again.
Supt Andy Hough, of Derby police, said: "It's very disappointing to see so many premises selling alcohol to underage people.
"We will be carrying out similar operations in the future and if the premises that failed this time fail again, we will take formal action against the licence holders."