The BB rat spontaneously develops insulin-dependent diabetes mellitus of autoimmune aetiology. From breeding studies, one of the genes necessary for the development of diabetes in these animals is linked to RT1, the rat's major histocompatibility complex. To study further the RT1 linked diabetogenic gene of the BB rat, we have studied restriction fragment length polymorphism using 32P-labelled DNA probes of the major histocompatibility complex genes. As we have previously reported, an I-A alpha probe (mouse class II gene) defines four chromosome types in the control BBN population, only one of which is found among diabetes prone BB rats. All BB rats we have studied are homozygous for the type IIa chromosome. Here we examine restriction fragment length polymorphisms using three other DNA probes. Using a DC beta-probe (human class II), the same pattern of polymorphisms (though different molecular weights) is found as with the I-A alpha probe. An H-2d C4 (fourth component of complement, mouse class III) defines no polymorphisms among or between BB and BBN rats. Using H-2 LdC-2 domain probe (mouse class I) many polymorphisms are apparent and in a limited series distinguishes I-A alpha defined IIa/IIa BBN rats from IIa/IIa BB rats. These studies provide the basis to subtype the RT1u identical BB and BBN animals and should aid in the localization and characterization of the RT1 linked diabetogenic gene of the BB rat.