Resting murine B lymphocytes can present rabbit anti-Ig to T cell lines specific for normal rabbit globulin. The T cell-B cell interaction is major histocompatibility complex (MHC)-restricted, and leads to activation, proliferation, and differentiation of the resting B cell into an antibody-secreting cell. Efficient antigen presentation and B cell activation depends upon binding of rabbit globulin to (membrane) mIg. To investigate the role of mIg in this polyclonal model for a T cell-dependent primary antibody response, we determined whether crosslinking of mIg is required either for efficient antigen presentation, as measured by helper T cell activation, or for the B cell response to T cell help, since all the direct effects of anti-Ig on B cells require crosslinking of mIg. We found that monovalent Fab' fragments of anti-IgM or anti-IgD work as efficiently as their divalent counterparts. Therefore, a signal transduced through the antigen receptor seems not to be required when T cell help is provided by an MHC-restricted T helper cell recognizing antigen on the B cell surface. Moreover, rabbit globulin bound to class I MHC molecules in the form of anti-H-2K also results in efficient antigen presentation and T cell-dependent B cell activation. However, mIg still appears to be specialized for antigen presentation, since anti-Ig is presented about three- to fivefold more efficiently than anti-H-2K.