People in 120 homes in Wisbech had been told to boil their water "until further notice" after supplies were contaminated.
Anglian Water said tests were carried out following repairs to a burst main near Blackbear Lane, Wisbech on Sunday.
The results showed "naturally occurring organisms that should not be there".
The firm had advised people to boil and cool all drinking water as "a precautionary measure".
A spokesman for Anglian Water said: "We haven't found anything harmful to human health but the harmless organisms are indicator species that something might have been there that should not be."
Homes in Blackbear Lane, numbers 2 to 74, all properties in Pendula Road, Lucombe Drive, Oaklands Drive and Lebanon Drive were affected.
A notice sent to affected customers said: "As an additional precaution, we are also making a small increase to the amount of chlorine in the water and carrying out some localised flushing.
"We apologise for any inconvenience, but stress that the health and safety of our customers must come first."
Householders in Cambridgeshire who have had to boil their water since Sunday have now been told it is safe to drink once again.