We have used a combined serologic and structural approach to study the distribution of I-region associated (Ia) antigens in nine strains of inbred and partially inbred guinea-pigs. All of the inbred strains studied with the exception of strain 2 animals were found to share one or more I-subregions with inbred strain 13 animals. The BIOAD, R9, OM3, and BIOAC strains have the same I-region as strain 13 animals; the B/Lac strain has two subregions in common with strain 13, while the BIOB strain has a single subregion in common with strain 13. The availability of a number of different guinea-pig strains with well-characterized major histocompatibility complexes should facilitate the continuing use of this species in studies of immunogenetics, transplantation, and tumour immunology.