The Regina Qu'Appelle Health Region says someone at the Pasqua Hospital laboratory probably added the wrong product to a tissue-processing machine.
More than 100 samples were negatively affected but about half of them could be salvaged.
The Health Region says a discussion with doctors determined that 10 to 15 patients might require another biopsy.
The samples are used to determine the diagnosis for a number of health concerns, including cancer, inflammatory, infectious or other medical conditions.
The health region says doctors of any of the patients whose samples were affected may choose to re-order the tests if they are concerned.
"We very much regret the impact this error has for the affected patients," said Dr. Ted Alport, the health region's head of laboratory medicine. "We have never had a similar incident."
Officials believe the chemical added in error was cytology clarifier, which a news release from the Health Region says contains acetic acid and alcohol.
The specimens had been delivered to the Pasqua Hospital from a number of communities in southern Saskatchewan. The error was identified on Aug. 23.
Alport said the health region has reviewed procedures and taken steps to improve identification and labelling of the containers involved. He also said the chemical in question is no longer stored near the processing machine.
Alport said even though more than 50 samples were ruined, many doctors determined there was no longer a need to get a new sample. One example, he said, involved an acute appendix — where a second sample cannot be obtained.