Around 1,000 children in Britain are among the first to be given a vaccine for swine flu.
Over the next two weeks the kids will be testing out which drugs work best to protect against the illness.
The idea is that everyone will be given this medicine to stop them from catching swine flu.
The children aged between six months and 12 years are being tested at five different places Oxford, Bristol, Southampton, Exeter and London.
Scientists will be looking at the results between two different drugs to see which one should be given out to the rest of the population.
The UK government has ordered around 60 million doses of the swine flu vaccine and are expecting to start giving it to people in October.