Our response focused on the steps that need to be taken to optimise the safety of phase I clinical trials involving very novel biological molecules, new agents with a highly species-specific action and new drugs targeted towards the immune system. These groups of drugs offer great potential, but due to the limited previous experience of the development of such drugs, exceptional care must be taken in designing phase I clinical trials, particularly with with regard to the initial dosing and the time intervals between doses of a drug in an individual and between individuals.