A large panel of enteric organisms was screened for agglutination with a panel of λ monoclonal antibodies of different heavy chain isotypes specific for α1-3 dextran (DEX). Two strains were initially isolated that were bound by most of the anti-DEX antibodies. One organism, Enterobacter cloacae strain MK7, which was characterized in detail, induced a typical λ anti-DEX response in Igh-Ca mice that had a fine idiotype profile comparable with that induced by purified B1355S dextran containing α1-3 glucosidic linkages (α1-3-DEX). The determinant on the bacterial surface was shown by binding inhibition with nigerotriose to contain α1-3 linkages. Hyperimmunization with these organisms of normal, athymic (nu/nu), or germ-free mice induced large amounts of IgM antibodies but very little IgG. This is the first description of an organism isolated from the normal gut flora of mice that can be shown directly to be bound by α1-3-DEX antibodies and to induce the typical germ-line response of the DEX family of antibodies.