Of 212 patients undergoing total hip replacement who were tested preoperatively for sensitivity to nickel, chromium, and cobalt, fourteen showed sensitivity. No sensitivity to the bone cement was recorded. Of 173 patients who had never had a metal implant before, 5.8 per cent showed a positive reaction. Two patients out of fifteen with a failed McKee-Farrar prosthesis were found to be sensitive. Of six patients who had a stable McKee-Farrar prosthesis in the other hip, none showed sensitivity to the metal. In four of sixty-six patients, sensitivity to nickel or cobalt developed after the implantation of a metal-to-plastic prosthesis. No patients were sensitized to the cement. The consequences of proved hypersensitivity in patients with metal-to-plastic prostheses, either present prior to insertion of the prosthesis or evoked by the implant material, are not known.