Mice primed with a thymus- (T)3 dependent form of Type III pneumococcal polysaccharide (S3), i.e., S3 coupled to sheep or horse erythrocytes (S3-RBC), produce S3-specific IgG antibody after secondary challenge with either the T-dependent (S3-RBC) or T-independent (S3) form of the antigen. The potential to produce IgG antibody after challenge with S3-RBC appears earlier after priming than the potential to produce IgG after challenge with S3, suggesting that different "memory" cells may be involved in the two responses. The "memory" cells were shown to be S3-specific since S3 had to be present on the carrier in order for priming to occur and carrier specificity was not required for elicitation of the secondary response by S3-RBC.