Monoclonal antibodies (MAbs) have traditionally been produced in quantity by transfer of the hybridomas to the peritoneal cavity of mice and subsequent recovery of MAb from the ascitic fluid. Legislation in many countries, however, now limits the use of animals for such purposes. Although it is possible to produce MAbs in vitro by maintenance of hybridomas under standard cell-culture conditions, this requires the processing of large volumes of supernatant and is expensive. This chapter describes a simple method for culturmg hybridomas in vitro in renal dialysis cartridges, which allows several hundred milligrams of MAb to be recovered at high concentration.