The structure and function of HLA cell membrane molecules and their function as histocompatibility molecules is reviewed in the article. The mechanisms responsible for the alloactivation of T cells being stronger after transplantation from an HLA-identical donor than following transplantation from an HLA-incompatible donor are also briefly discussed. The beneficial effects of HLA compatibility are illustrated by the 10-20 per cent higher survival rate of HLA matched grafts, as compared with HLA mismatched grafts, found in a Norwegian series of living and cadaveric donor kidney transplant material. The effects of HLA compatibility on heart, lung and pancreas transplants are also briefly reviewed. It is concluded that the more clearly demonstrable effects of HLA matching now seen are at least to some extent due to better HLA typing techniques, but that more information is needed concerning acceptable and non-acceptable HLA mismatches, the effect of pre-sensitization and methods for inducing specific tolerance to donor antigens.