All the biologically relevant HLA class II allelic variants can not be identified with conventional serological tissue typing techniques. During the past few years considerable advances have been made in HLA class II typing with molecular techniques now widely used in routine clinical tissue typing. The next few years are likely to see the development of tissue typing techniques based on the polymerase chain reaction (PCR), for use in acute transplantation. A new method for the rapid identification of genetic polymorphisms is described--allele-specific PCR amplification.