A direct comparison was made between thymus-dependent (TD) and thymus-independent (TI) responses in mice tolerized for (1 leads to 6) glycosyl determinants by the injection of dextran B512. Long-lasting B-cell tolerance by dextran was reversed when mice were treated with dextranase in vivo. Complete or partial reversion of tolerance with the enzyme was invariably obtained for the TI response but the TD component proved to be more resistant and dependent on the immunogen used to test the reversion. The uniformity of the spectrotype in BALB/c mice, even under conditions of partial tolerance, permitted the analysis by isoelectric focussing of serum from tolerant mice treated with dextranase and immunized with TD dextran-ovalbumin. Results showed that, with one single exception, mice thus treated produced spectrotypes no different from the pattern normally found in immune animals. The results presented suggest that at least some alpha(1 leads to 6) specific B cells, both TD and TI, persist in tolerized mice for at least 2 weeks after tolerance induction and they do not support the concept of clonal elimination for either TI or TD responses in adult mice.