Although it is well established that B lymphocytes are able to present antigen in vitro, the ability of small resting B cells to act as antigen presenting cells in vivo remains controversial. In this report we have studied the antigen presentation and the antibody response induced by mouse B cells after in vivo or in vitro targeting antigens to membrane Ig (mIg), using rat mAbs. Our results show that injection of these mAbs coupled to 2,4-dinitrophenyl (DNP) strongly enhances the IgG1 antibody response against DNP and rat Ig. T cell depleted spleen cellspulsed in vitro with rat Ig without specificity for B cells induced an antibody response when re-injected into mice, this response being much higher if the antigen was specific for mlg. Moreover, purified resting B cells were shown to induce a specific IgG1 response in vivo only when they were cultured with rat mAb against mIgM or mIgD but not with myeloma rat Ig of the same isotype. B cells do not need to be activated to present antigen since the induction of the specific antibody response does not correlate with the mitogenic activity of rat mAb nor with the IgG1 polyclonal synthesis in vivo. These data clearly show that resting B cells can present antigen in vivo and induce an antibody response, and underline the importance of mIgM and mIgD as targets for antigens.