Transfer of live lymphoid cells from BGG-immunized strain 2 guinea pigs into syngeneic animals primed with DNP-OVA prepares the recipients for a markedly enhanced secondary anti-DNP antibody response upon challenge with DNP-BGG. This phenomenon has been demonstrated when the recipients were challenged 1 day after cell transfer, but it was considerably more striking when an interval of 6 days was allowed between transfer of cells and challenge with antigen. As demonstrated in the preceding paper (6), BGG preimmunization enhanced anti-DNP antibody responses to challenge with DNP-BGG. An analysis of the characteristics of substances to which preimmunization was effective in enhancing subsequent anti-hapten responses was made. It was shown that preimmunization of strain 13 guinea pigs with a copolymer of glutamic acid and lysine (GL), to which these animals are genetically unable to accord an immune response, failed to prepare them for an enhanced anti-DNP response to DNP-GL. Tolerance to BGG specifically abrogated the capacity of subsequent BGG immunization to prepare DNP-OVA primed rabbits for an enhanced anti-DNP response to DNP-BGG. Sensitization of animals to haptens by preimmunization with hapten-protein conjugates failed to prepare them for enhanced primary or secondary antibody responses to other determinants associated with that hapten on a different carrier. These studies indicate that the enhancing effect of carrier preimmunization reflects a cooperative interaction between lymphoid cells specific for carrier and cells specialized for haptenic determinants. Furthermore, the capacity of a substance to behave as a carrier requires more than its possession of a variety of antigenic determinants.