Eight 15I₅ B-congenic White Leghorn chicken lines, containing haplotypesB ² , B ⁵ , B ¹², B ¹³, B ¹⁵ , B ¹⁹ , and B ²¹, were subjected to molecular genotyping with chicken B-F (Class I) and B-L (Class II) major histocompatibility complex (MHC) probes. Genomic DNA was digested with restriction enzymes, hybridized with a Class I or Class II probe, and analyzed for restriction fragment length polymorphisms. Digestion withHindIII or EcoRI yielded no B-L polymorphisms. Digestion with PvuII or BglII and hybridization with a B-L or B-F probe produced polymorphisms that were shared between several haplotypes, although the haplotypes with similar patterns were clustered differently between Class I and Class II probes. The genetic variation seen for B-L and B-F probe hybridization ofPvuII digests was much less than that previously demonstrated for B-Gprobing of PvuII digests of the same lines. Description of MHC Class I and II restriction patterns of the well-characterized 15I₅ B-congenic lines will aid in identification of genes important in disease resistance.