To determine whether or not B lymphocytes are committed to the synthesis of a single immunoglobulin heavy chain isotype during their differentiation into plasma cells, rabbit lymph node and Peyer's patch cells were separated into populations with and without membrane IgM, using a fluorescence-activated cell sorter (FACS). The potential of the micro-bearing (micro+) and non-micro-bearing (micro-) cells to give rise to plasma cells both in vivo after transfer into irradiated recipients and in vitro in the presence of pokeweed mitogen was assessed by immunofluorescence techniques, and the relative proportions of the cytoplasmic Ig-stained cells (CSC) synthesizing each class of heavy chains were determined. Most of the CSC arising in vitro from micro-bearing lymph node and Peyer's patch cells contained IgM; all IgM CSC appeared to be derived from micro+ cells. Peyer's patch lymphocytes, however, did not generate IgM CSC after cell transfer and thus may be functionally different from lymph node micro+ cells. It was found also that nearly all of the many IgA CSC generated by Peyer's patch lymphocytes either in culture or after transfer were derived from micro- cells. Further fractionation of these micro- cells with the FACS after they had been membrane stained with anti-b locus allotype reagents revealed that the precursors of IgA CSC belong to a minor population of cells which do have b locus light chain determinants on their membranes, although they do not have detectable micro-chains. These cells are not found in lymph nodes. Although the majority of Peyer's patch and lymph node cells were found to be precommitted to the synthesis of a single heavy chain isotype, a small proportion of cells may not be similarly restricted. Some of the CSC with membrane IgM were found to contain cytoplasmic IgA or IgG. In addition, micro+ populations did give rise to low numbers of IgA and IgG CSC. The implications of these results, obtained under experimental conditions, on the normal differentiation of B lymphocytes in situ are discussed.