We evaluated MHC class I- and II-restricted presentation of exogenous antigen by mouse bone marrow-derived dendritic cells (DC) and splenic B cells. DC presented to class I-restricted transgenic T cells femtomolar concentrations of antigens from liposomes targeted to the IgG Fc receptor. Targeting these liposomes to surface immunoglobulin did not permit B cells to stimulate class I-restricted responses. Nevertheless, both DC and B cells presented antigen from liposomes targeted to these same receptors with equivalent efficiency to class II-restricted T cells. Acquisition of the capacity to present class II-restricted antigens required shorter periods of differentiation of DC than presentation of exogenous class I-restricted antigens. The latent period for class I-restricted presentation of exogenous antigen by DC could not be shortened by exposing them to lipopolysaccharide, double-stranded RNA or antibody to CD40. Class I presentation depended on expression of the TAP1 transporter. Our data are consistent with the existence of a regulated transport process present in DC which can convey exogenous antigen from endocytic vesicles to the cytosol.