Durham Police say they are disappointed at the number of pubs and clubs in the county which are willing to serve underage drinkers.
Since March, staff at 28 licensed premises in North Durham have been issued with fixed penalty fines for selling alcohol to teenagers.
Most recently, three pubs in the Stanley area sold alcopops to two 15-year-old schoolgirls.
In each case, bar staff failed to carry out simple age checks, said the force.
Last week, at the Sun Inn and the Burton House, both in Burnopfield, and Stanley's Causey Equestrian Centre, the schoolgirls were served without question.
The bar staff responsible were each given on-the-spot fines of £80.
Durham Police say they will be carrying out further operations to clamp down on underage sales, particularly in the run-up to Christmas.
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