A detailed understanding of the structure and function of the human major histocompatibility complex (MHC) has ensued from studies by molecular biologist during the last decade. Virtually all of the HLA genes have now been cloned, and the nucleotide sequences of their different allelic forms have been determined. Typing for these HLA alleles is a fundamental prerequisite for tissue matching in allogeneic organ transplantation. Until very recently, typing procedures have been dominated by serological and cellular methods. The availability of cloned DNA from HLA genes has now permitted the technique of restriction fragment length polymorphism (RFLP) analysis to be applied, with remarkable success and advantage, to phenotyping of both HLA Class I and Class II determinants. For the HLA Class II genes DR and DQ, a simple two-stage RFLP analysis permits the accurate identification of all specificities defined by serology, and of many which are defined by cellular typing. At the present time, however, RFLP typing of HLA Class I genes is not as practicable or as informative as that for HLA Class II genes. The present clinical applications of HLA-DR and DQ RFLP typing are predominantly in phenotyping of living donors, including selection of HLA-matched volunteer bone marrow donors, in allograft survival studies, and in studies of HLA Class II-associated diseases. However, the time taken to perform RFLP analysis precludes its use for the typing of cadaveric kidney donors. Nucleotide sequence data for the alleles of HLA Class II genes have now permitted the development of allele-specific oligonucleotide (ASO) typing, a second category of DNA analysis. This has been greatly facilitated by the ability to amplify specific HLA Class II DNA 'target' sequences using the polymerase chain reaction (PCR) technique. The accuracy of DNA typing techniques should ensure that this methodology will eventually replace conventional HLA phenotyping.