HLA typing in 1979 is more complex and more complete than in former years. The distribution of antigens in the HLA system and various methods of identifying them and of determining their structure and function are described. The usefulness of HLA typing in organ transplantation and paternity testing has been well established, and its place in blood component therapy is to some degree still conjectural. Detection of HLA antigens associated either positively or negatively with specific disease states has great importance for several reasons. From an academic point of view, these association may help to define pathogenetic mechanisms, and from a practical viewpoint, there is now good reason to believe that HLA typing may shortly be useful as a diagnostic and predictive tool. Further elucidation of the role of the HLA molecules in the immune response gives hope for better understanding of the immune and autoimmune disorders, which is surely the first step in defining rationally based therapeutic regimens.