Twelve C57BL/6J anti-B-1355S monoclonal antibodies (five IgM lambda and seven IgM kappa) were characterized immunochemically by binding and inhibition ELISA. All 12 were negative for the expression of the cross-reactive idiotype (IdX) of BALB/c mice, as expected from previous work; no kappa IdX+ antibodies have been reported and IdX- lambda class antibodies were observed in B-1355-Con A induced immune sera [Geckeler W., Blomberg B., dePreval D. and Cohn M. (1977) Cold Spring Harb. Symp. quant. Biol. 41, 743-748]. The antibodies studied bind to B-1355 coated plates and this binding is inhibited by B-1355 but not by dextrans B-512 (F) or B-742S; the latter two have no linear alpha (1----3; 1) linkages. The nomenclature of Jeanes is used [Jeanes A. (1986) Molec. Immun. 23, 999-1028]; alpha (1----3; 1) refers to glucosyl diose residues linked alpha (1----3) linearly. In the case of B-1355 these linear stretches alternate with alpha (1, 6) linkages and are non-contiguous; alpha (1----3; b) refers to the linkage at a branching residue, e.g., a 1,3,6 linked moiety. The IgM kappa class antibodies are not inhibited by nigerose or nigerantetraose, suggesting that they have binding site sizes which are unusually large for B-1355 specific antibodies. The five IgM lambda antibodies are inhibited identically by equimolar amounts of nigerose and nigerantetraose, suggesting that their binding sites accommodate a disaccharide epitope. These antibodies are also inhibited by the alpha (1----6), alpha (1----4) triose, panose. The kappa class antibodies do not bind to alpha (1----3)-diglucosyl-(nigerosyl; N)-BSA. Four of the five lambda class antibodies show weak binding to N-BSA, while the fifth binds N-BSA better but less well than MOPC 104E (the BALB/c myeloma protein). All 12 antibodies are unique when compared to BALB/c antibodies derived from B-1355 immunization. The primary response of 15 C57BL/6J mice to B-1355 was re-assessed for kappa and lambda class antibody contribution. A patchy lambda class response was observed suggesting that previous lambda class responses may have been overlooked.