The serological systems of the rabbit which are under specific gene control are the subject of this investigation. We are interested in the gene-antigen relationship, particularly in the study of the mechanisms responsible for the appearance of antigens with unusual modes of inheritance, such as the interaction antigens. The rabbit blood group systems now consist of 7 loci controlling 21 antigens. One locus, the Hg locus, has 4 alleles controlling 12 immunologically specific antigens. This latter system is also the subject of studies of the inheritance of antibody responsiveness and for studies relating to problems in the maternal-fetal relationship.