We investigated the possibility that immunoglobulin (Ig) receptors facilitate uptake and processing of exogenous antigens (Ags) via class I pathway in B cells. B-cell hybridomas were transfected with a plasmid containing structural genes of n and K chains of IgM specific for trinitrophenyl (TNP). The B-cell transfectants express membrane-bound but not secreted anti-TNP IgM. Using a hapten-carrier system, we found that ovalbumin (OVA) conjugated to TNP was efficiently taken up by B-cell transfectants and presented to CDS, class I MHC -restricted T cells in a dose-dependent manner, whereas native OVA was not processed by Bcell transfectants via class I pathway. By contrast, untransfected B cells did not present TNP-OVA or native OVA to class l-restricted T cells. The uptake of TNP-OVA by B-cell transfectants was blocked by anti-idiotypic antibody. Thus, B cells take up exogenous Ags into class I pathway via the receptor-mediated endocytosis. Moreover, class I processing and presentation of TNP-OVA by B-cell transfectants was inhibited by Brefeldin A and proteosome inhibitors, but not by chloroquine. We conclude that B cells expressing specific Ig receptors can also process and present soluble, exogenous Ags to class l-restricted T cells. Thus, B cells, like some phagocytic macrophages, pocess specialized methods for uptake and delivery of exogenous Ags into the class I processing pathway. Supported by AI-13987 and IM-617A grants.