The majority of B lymphocytes in adult mice express both IgM and IgD on their surface (sIgM and sIgD). A small percentage of sIgM+ splenic B cells lack (or express very low levels of) sIgD. These cells have been termed "mu-predominant" (mu p) B cells. In neonatal mice (5 to 12 days of age), mu p B cells account for more than 50% of the sIg+ cells. There is conflicting evidence concerning the immunocompetency of mup cells in vitro. To study this question further, splenocytes from neonatal BALB/c mice were depleted of sIgD+ B cells by a panning procedure. A portion of the nonadherent (mu p) cell population was analyzed for residual sIgD+ cells by using indirect immunofluorescence in conjunction with the fluorescence-activated cell sorter (FACS). Such cells were then tested for their responsiveness to the thymus-independent (TI) antigen, trinitrophenyl Brucella abortus (TNP-BA), by using a limiting dilution culture system. Results indicate that the depletion of sIgD+ B cells and the decrease in the precursor frequency of splenocytes responding to TNP-BA are very similar, suggesting that virtually all of the responding B cells bear sIgD.