Presentation of GAT by GAT-specific hybridoma B cell clones to GAT-specific T cell lines and clones was investigated. B cell clones expressed surface IgG2b, Ia and Fc receptors, and did not secrete immunoglobulins. The clones formed rosettes with GAT-SRBC which could be inhibited by monoclonal antibodies against GAT idiotype. Presentation of GAT by the clones was MHC-restricted and highly efficient, i.e., a few hundred B cells or very low concentration of GAT stimulated proliferation of GAT-specific T cell lines. The fusion partner, M12.4.5.2, clone presented GAT much less efficiently. Although GAT-specific B cell clones presented also heterologous antigen beef insulin to insulin-reactive T cell lines, they were not able to present insulin in the absence of soluble antigen. Presentation of GAT was inhibited by pretreatment of the clones with monoclonal antibodies against I-E, mouse Ig and GAT-idiotype, as well as related antigens GA and GT. The rate of antigen uptake was much shorter compared to resting B cells for cells pulsed with GAT for 4 hours presented the antigen well. The studies suggest that a membrane-associated Il-1 may be required in the presence of antigen presentation by B cells. The results also revealed that antigen-specific hybridomas can present specific antigen in part with association of surface immunoglobulins which may be involved in antigen uptake and focusing to T cells.