During ontogeny IgD appears later than IgM on splenocytes of neonatal mice (1) and at a time when mice develop a markedly increased immune responsiveness (2). Based on these observations, it was suggested that IgD serves as a "triggering" isotype for induction of immune responses, whereas surface IgM functions as a tolerizing receptor (3). To test this hypothesis, the susceptibility of adult splenocytes (which are predominantly mu(+)delta(+)[4-6]) and neonatal splenocytes (which bear predominantly IgM [mup(+); 1, 4-6]) to tolerance induction were compared. The results indicate that neonatal splenic B cells responsive to thymus dependent (TD) antigens are exquisitely susceptible to tolerance induction compared with those from adult mice (7-9). However, cells from both adult and neonatal mice were highly susceptible to tolerance induction when thymus independent (TI) antigen was used as immunogen (8). These results suggest that the major precursor for the TD response is a mu(+)delta(+)-cell which appears late in ontogeny and is resistant to tolerance induction and that the mup(+)-cell is the major precursor for the TI response and is highly susceptible to tolerance induction. Other differences between responders for TI and TD antigens have been described previously (10-12). To test this concept, adult splenocytes were treated with papain under conditions in which IgD, but not five other surface molecules, was removed (13). Such treated splenocytes were shown to be markedly susceptible to tolerance induction, resembling TD responders from neonatal animals. This experiment was interpreted as indicating that IgD confers resistance to tolerance induction on mu(+)delta(+)-cells. To prove this interpretation, it is necessary to show that specific removal of IgD with anti-delta also results in increased susceptibility to tolerance induction and that treatment with anti-mu does not have a similar effect. In the present studies, we have removed surface IgM or IgD by antibody-induced capping and assessed the tolerance susceptibility of the treated cells. Our results demonstrate that removal of IgD, but no IgM, from TD responders increases their susceptibility to tolerance induction.