A method is described that allows rapid purification of IgG monoclonal antibodies from tissue culture supernatant; it is easy, relatively cheap and produces a product of high purity. Protein G linked to Sepharose fast flow gel has a high capacity for binding IgG and permits large volumes of supernatant to be loaded at high flow rates. However, Protein G cannot discriminate between the monoclonal antibody and other irrelevant IgG e.g. bovine IgG present in the tissue culture supernatant. Therefore, bovine IgG was removed from foetal calf serum by passage over protein G before being used as part of standard tissue culture medium. Hybridomas producing a rat anti-mouse (e.g. KT6) and a mouse anti-mouse (e.g. KJ23a) mAb were cultured in such tissue culture supernatant depleted of bovine IgG. Subsequent passage of the spent culture supernatants over protein G produced good yields of mAb of high purity and concentration. This method can been used to produce many different mAbs without the need for growing cells in a live host as ascites, or the need for specialised tissue culture media or expensive chromatography equipment.