Immunization of certain strains of mice with the tobacco mosaic virus protein (TMVP) elicits antibodies capable of binding with a decapeptide representing residues 103-112 of TMVP. However, immunization of mice with decapeptide conjugated to a variety of carriers failed to elicit antibodies to the decapeptide in spite of the fact that antibodies were induced to the carriers or to nonrelated peptides similarily conjugated to the carriers. In contrast to the conjugates inability to induce a primary anti-decapeptide response, the conjugates were able to elicit a secondary anti-decapeptide response. Thus, when irradiated mice were transplanted with syngeneic TMVP-primed B cells and carrier-primed T cells, secondary challenge with the decapeptide on the carrier elicited an excellent secondary anti-decapeptide response. Furthermore, the titer and specificity of the antibodies so induced were the same as that induced by secondary challenge with TMVP. These results suggest a difference between primary and memory B cells in their requirements for activation.