A marked enhancement of anti-hapten antibody synthesis in response to immunization with a hapten-protein conjugate occurs as a result of pre-immunization of animals with unmodified carrier. This is true both for primary and secondary antihapten responses and is most strikingly demonstrated when an animal which has received a primary immunization with a hapten conjugate of a given carrier is boosted with the same hapten conjugated to a different carrier, to which the animal has been preimmunized (1-6).This phenomenon indicates the existence of independent recognition units for hapten and for carrier. Cooperative interaction between these units may be essential to the production of an anti-hapten antibody response, or may amplify a modest immune response which occurs in the absence of carrier-specific recognition.An understanding of the nature of the carrier recognition unit and of the range of substances which it recognizes and acts upon is required in order to construct a coherent picture of cooperative interactions in the immune response. Studies presented both by Mitchison and Rajewsky, et al. (4,5) and those reported in the first paper of this series (6) show that humoral antibody is not the carrier recognition unit.In the adoptive secondary antibody response in mice, hapten-carrier cooperative interactions appear to be mediated by hapten-specific and carrier-specific lymphoid cells (4, 7). In the present communication we show that live lymphoid cells from strain 2 guinea pigs immunized to bovine gamma globulin (BGG) enable syngeneic animals immunized with 2,4-dinitrophenyl ovalbumin (DNP-OVA) to form a secondary anti-DNP response to DNP-BGG. Thus, in both species the carrier recognition unit appears to be associated with a lympboid cell.Several relevant questions may then be asked concerning the significance of this phenomenon and the mechanism by which carrier-specific cells enhance anti-hapten antibody response. One suggested function of the carrier-specific cell is the concentration and presentation of antigen (4, 7). Such cells would bind multideterminant antigens by one of their determinants and simply present other determinants to appropriate hapten-specific cells ill certain anatomical locations. However, the possibility must also be considered that 283 on