A state of hypersensitivity to the metallic constituents of prosthetic implants can be determined by culturing the patients lymphocytes in vitro in a medium containing the appropriate concentration of salts of the metal. If in these circumstances the lymphocytes proliferate, the patient is hypersensitive. Chromium, cobalt and nickel salts can produce such mitosis only in the lymphocytes of hypersensitive patients; the same test produces no such reaction in control subjects. This hypersensitivity can cause delayed loosening of the prosthesis. The lymphocyte transformation test (L.T.T.) is therefore a very useful preoperative measure in detecting patients who are allergic to these metals and so enables the surgeon to choose the right implant.