Much of the 25 years since Kohler and Milstein first described making monoclonal antibodies (mAbs) has been spent trying to develop these reagents to treat human disease. Until recently, progress has been frustratingly slow and by 1994 only one mAb, anti-CD3 (OKT3), had been licensed for clinical use. In the past five years, however, the situation has changed dramatically, with numerous mAbs now showing clinical potential, and a further seven approved for human treatment. Furthermore, all indications are that this upward trend will continue, with a quarter of all new biological products currently undergoing clinical development being antibody based.