Stocks of beef burgers have been removed from school kitchens across North Lanarkshire as a precaution, the council said.
The discovery was made after frozen burgers were removed for testing last week. Samples of frozen mince came back negative for horse DNA.
A local authority spokesman said: "The council has notified the Food Standards Agency, as it is required to do, and investigations are continuing.
"Our investigations are focusing on the use of frozen burger supplies during the past three months, the maximum length of time these would be held in storage."
It did not identify the school kitchen involved.
Meanwhile, Renfrewshire Council said it has removed some meat products from school kitchens and other catering premises until investigations into horse meat tests are complete.
Bosses said they were taking the step as a "precaution" after one of its suppliers, the Brakes Group, recalled lasagne produced for another customer in order to test it for horse DNA.
A council spokesman said: "No horse DNA has been found in any of the Brakes Group products supplied to Renfrewshire Council.
"While there is no risk to human health, we have removed some Brakes Group products from our catering premises as a precaution until the results of the investigation are known."
Some beef products were withdrawn from restaurants and cafes in the House of Commons last week as they were supplied by Brakes.
A statement from the company said the first batch of tests have proved negative for horse meat.
It said: "Brakes can confirm that it has received the result of a DNA test for the Brakes-brand lasagne product it recalled as a precaution at the end of last week. The product has tested negative for horse meat DNA.
"We would like to apologise to customers who were inconvenienced but we felt that this was the most appropriate course of action at the time."
The scandal came to light last month when the Food Safety Authority of Ireland announced the discovery of equine DNA in some beef burgers.
The Holyrood Environment Secretary Richard Lochhead has said it could be a watershed moment with positive consequences for consumers.
Renfrewshire Council said its food suppliers are sourced through Scotland Excel, which deals with contracts on a national basis.
Other councils have also tested products in the wake of the scandal.
Fife Council leader Alex Rowley said there is "no risk" of school meals in the region containing traces of horse meat while Tayside Contracts, which caters for schools across Dundee, Angus and Perth and Kinross, said there was "no evidence" to suggest its meals contained horse meat.
Borders Council said it has been assured by suppliers that they do not use any of the companies so far affected affected by the situation while Edinburgh City Council said it had checked school dinners and found no samples that caused concern.
Shetland Council said schools and care homes are supplied with meat rather than processed meals, and tests showed it was not affected.
Moray and North Ayrshire councils also said they source meat locally and prepare meals on site.
West Dunbartonshire said food safety officers were carrying out inspections on four meat production plants in the area but that meals supplied to school canteens and council care homes is not affected.