We explored the possibility that wear products of cobalt-chromium alloy might lead to sensitivity to metal wear products and in turn to loosening of a component of the prosthesis after total joint replacement. Twenty patients with sterile, loose McKee-Farrar hip replacements had patch tests for sensitivity to cobalt, nickel, and chromium. All tests were negative in all patients. The histological findings from surrounding tissues in seventeen patients who had reoperation showed no signs of delayed hypersensitivity. In five patients, lymphokine assays for migration inhibition factor and blastogenic factor were done. Only one assay was positive. Our findings do not support the suggestion that hypersensitivity to metal is a cause of component loosening after McKee-Farrar total hip replacement.