An improved method for the detection of DNP-specific PFC was used for studying the kinetics of the immune response to the DNP-group in mice. DNP-specific PFC were already detected in the spleens of immunized mice after the first challenge. Direct PFC predominated on days 4 and 6, but by the 8th day a conversion to predominantly indirect PFC had occurred. A much stronger immune response was reached after the second challenge when the number of indirect PFC was consistently higher than that of the direct PFC. The immune response after the third challenge did not differ significantly from the response after the second challenge. The number of background plaques against SRBC remained low following both the first and second challenge. An attempt was made to induce tolerance to the DNP determinant by the injection of soluble DNP-MGG. However, the immune response in the pretreated group paralleled that of the controls, and the differences in the number of PFC or HA titers were not sufficient to warrant the conclusion that tolerance had been achieved.