Five cattle and five sheep that had never been exposed to bluetongue virus (BTV), as well as ten animalsthat had been experimentally infected with BTV-8, were inoculated with BTV-1. Previous exposure toBTV-8 did not prevent a second infection with another serotype.After the experiment, the BTV-1 preparation was found to be contaminated with BTV-15. The inoculumand blood samples taken during the experiment were analysed by serotype-specific real-time RT-PCR.There was 100-fold less BTV-15 than BTV-1 in the inoculum. Unexpectedly, BTV-15 dominated the infectionin cattle that had previously been exposed to BTV-8. In sheep of both groups, on the other hand,BTV-1 prevailed over the contaminant.Regardless of the outcome, the incident demonstrates the need for a thorough contamination screeningof virus preparations. For this purpose, two type-specific RT-PCR primer sets for each of the 24 establishedBTV serotypes as well as Toggenburg Orbivirus were designed.