Officials at the Bowie Training Center said blood samples from a euthanized horse at the Bowie Training Center show the animal was not infected with equine herpesvirus.
A hold order placed Saturday on 27 horses in Barn 16 at the training center has been lifted.
Horses stabled in the barn will be allowed to train beginning Wednesday morning.
Racing officials say a horse stabled at Barn 16 was put down Saturday morning after it was suspected of having the illness.
Equine herpesvirus-1 causes upper respiratory infection and can cause neurological disease.
The illness can spread by coughing or sneezing, as well as by direct contact with infected horses, feed and equipment.