The Czech Republic veterinary authority has said horse meat imported from Poland tested positive for phenylbutazone, a drug given to horses that can cause health problems in humans.
Horse meat from animals treated with the drug is banned from being sold in the European Union.
Josef Duben, spokesman for the authority, said the drug was detected in correctly labelled horse meat on sale in a store in the eastern city of Ostrava, near the Polish border.
Authorities are aware of about 660lb of the tainted meat delivered from Poland to the store.
Thousands of meat products are being tested for the drug, and for horse DNA, after horse meat was found in food products labelled as beef or pork across Europe.