The “splenic focus” technique has been extended to an analysis of the focal response in vitro to both carrier and haptenic determinants of the 2,4-dinitrophenylhemocyanin (DNP-Hy) molecule. The response was shown to depend on the use of previously immunized donor cells and was directly related to the number of such cells injected. While anti-DNP foci could be stimulated with the hapten on heterologous carriers, the number of foci was maximized by the use of DNP-Hy, the complex used for primary immunization of the donor cells. Foci producing anti-DNP antibody were detected in fragments producing no detectable anti-Hy antibody; however, more fragments were found with foci producing both anti-DNP and anti-Hy antibody than would have been expected if stimulation were by two determinants which were independent of one another.