The hapten‐protein conjugate, 2,4‐dinitrophenyl‐hemocyanin [DNP‐KLH], is highly immunogenic when bound to macrophages of normal mice. Indeed, such macrophages effectively stimulate a strong anti‐hapten response if presented to lymphocytes from mice previously immunized to the same hapten protein conjugate. On the other hand, the lymphocytes will not respond if the macrophage contains the hapten bound to an unrelated carrier, such as bovine IgG. In these experiments we tested whether such lymphocytes would respond immunologically to the hapten and carrier determinants, if these were in different molecules, but presented on the same macrophage. We have found that lymphocytes from DNP‐KLH immune mice respond weakly to macrophages that contain both the hapten DNP on bovine IgG plus hemocyanin. Hence, the optimal T and B lymphocyte collaboration via macrophages demands that both the haptenic and carrier determinants be on the same molecule.