The therapeutic potential of monoclonal antibodies (mAb) was quickly realised after the hybridoma technique allowed their development in the mid 1970s. Chimeric humanised and fully humanised mAb can now be made by recombinant engineering. About a quarter of all biotech drugs in development are mAb, and around 30 products are in use or being investigated. Licensed products are available for inhibition of alloimmune and autoimmune reactivity, and for antitumour, antiplatelet, or antiviral therapy. Short-term side-effects are tolerable and as expected, although long-term safety remains to be elucidated. The cost-effectiveness and quality-of-life benefits of the use of mAb in patients who are usually seriously and chronically ill also needs studying. The therapeutic use of mAb is now established, and is perhaps the first example of how the "new biology" and the understanding of underlying molecular mechanisms has benefited patients.