Correlation of leukocyte groups with skin and renal allograft survival indicates that ranks of histocompatibility based upon current genetic concepts of the HL-A system may provide an approach to the selection of optimally compatible subjects for clinical organ transplantation. Such ranks may be expressed as a net histocompatibility ratio (NHR) between prospective donors and recipients. The best clinical results have been when this ratio is of 0.5 to 1. Donor-recipient compatibility situations where the ratio was 0.25 or less have been associated with a high incidence of transplant failure, regardless of whether the organ source was a living, related donor or a cadaver donor.