Six clinical centers recruited high-risk patients and collaborated with their local eye banking and organ procurement agencies in procuring donor corneal tissue. For each of the two studies, a total of 400 patients were sought. Blood samples from each enrolled patient were sent to the local CCTS tissue typing laboratory for HLA typing, and serum samples were sent to the Central Laboratory to be screened for preformed lymphocytotoxic antibodies. Depending on the results of the testing, patients were entered into the Crossmatch Study or the Antigen Matching Study.