The development of therapeutic monoclonal antibodies as drugs is characterised by a critical step, where a GO or no GO decision is made, following thorough validation of the drug candidate, a process that involves functional assays and in vivo biological models. This validation, as much as possible, must anticipate the medical relevance and efficacy of the antibody. Much of the subsequent rate of clinical failure can be attributed to inadequate validation at this critical GO/NO GO step.