The structure of major histocompatibility complex class I (HLA-A, B and Cw) and class II (HLA-DP, DQ and DR) genes and their products is now well understood. Knowledge of the high degree of polymorphism which occurs at all MHC gene loci, primarily derived from serological studies, has largely been confirmed and extended by the application of functional, biochemical and molecular techniques. The frequencies of class I and class II antigenic products are variable between ethnic groups and also between many diseased and healthy populations. In many cases the occurrence together of two or more MHC gene products (allelic association) provides haplotypic markers specific for diseases. The function of MHC products is widely speculated upon but little is known of the precise mechanisms whereby class I and class II molecules mediate their prescribed functions of co-ordinating cell-cell interactions, their involvement in susceptibility to disease or their established role in the artificial situation of organ allografting.