The horses were exported to Spain among other countries, the prosecutor said, without naming the others.
Sanofi told Le Parisien newspaper on Monday that the horses were used to create antibodies against rabies and tetanus among others. The company, which said it cooperated in the investigation, said it has resold about 200 horses in the past three years to vet schools, individuals and professionals.
The company said the lab uses horses for about three years before re-selling them, tagged and certified. It said it does not carry out testing on the animals, only makes lifesaving medicines.
Benoit Hamon, France's consumer affairs minister, told RTL radio, "These were horses that should have ended up at the slaughterhouse, and instead they ended up at the butcher."
He drew a sharp distinction between Monday's raids and a scandal earlier this year in which inexpensive but edible horse meat was passed off as beef and sold in supermarkets and restaurants around Europe.
"There are horses that should end up neither on your plate nor at the butcher, and that's the work of this investigation," he said.
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Angela Charlton in Paris contributed to this report.
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