Measurement of the serum IgM response against the T-cell–independent antigen NP-Ficoll. Results are for individual mice with (□) the preimmunization titer and (▩) the postimmunization titer. All titers were normalized to the average preimmunization titer obtained for the CBA/J control mice. Groups 1 and 2 received 5.0 × 10⁶ total cells, of which 5% or 0.5% were normal, respectively, whereas groups 3 and 4 received 0.5 × 10⁶total cells, of which 5% or 0.5% were normal, respectively. Responses seen in xid (xid) and wild-type (WT) control mice are indicated.