Two teenage girls were sent to buy alcohol from pubs and off licences in Morpeth and Bedlington, by officers hoping to catch those who serve drinks to under-age customers.
The volunteers - aged 14 and 16 - were served alcohol in the Market Tavern pub in Bedlington. The staff member who served them was fined £80 and management will also be fined.
In the Seen nightclub in Morpeth, both girls were allowed to enter the club and one of them was served alcohol.
Neighbourhood Inspector David Simpson from Northumbria Police said:
– Inspector David Simpson
"I'm very disappointed that licensed premises continue to serve alcohol to those who are underage.
"Some of the premises we visited though were thoroughly professional and refused to serve the teenagers.
"We will continue to target underage drinking in the local area and action like this sends a strong message out to all licensees that selling to children or allowing kids to drink will not be tolerated here in Northumberland."