A human-human IgM (lambda) hybridoma antibody (called Tr7E2) was constructed by fusing Epstein-Barr virus-transformed cells from a multiparous woman with the human fusion partner KR12. By eosin exclusion microcytotoxicity the monoclonal antibody killed 12 of 13 human leukocyte antigen DQw1-bearing lymphoblastoid cells. No reaction was seen with any of 19 DQw1-negative cells. The single DQw1+ cell line that was not killed by Tr7E2 was the homozygous cell called 9WS 806 TAB (DR8,8; DQw1,1) of Japanese origin. The radioimmunoassay indicated that this result was probably not because of a decreased expression by this cell of DQ antigens, and these cells were killed by the mouse monoclonal antibody Genox3.53G2a5, reported to be specific for DQw1. Thus, the Tr7E2- cell line TAB probably expresses a novel structural DQw1 variant. Of 213 Norwegians, 107 were Tr7E2+ Genox+; none expressed only one of these epitopes. The putative split is, therefore, probably very rare in this population. Monodisperse magnetic polymer beads coated with Tr7E2 formed rosettes selectively with peripheral blood mononuclear cells from DQw1-positive individuals, suggesting a new approach to typing for class II antigens.