Guinea pigs fed dinitrochlorobenzene (DNCB) in corn oil become unresponsive to sensitization by later percutaneous applications of DNCB in olive oil. Sera taken from animals after feeding DNCB or after feeding and intradermal stimulation with DNCB contained no antibody detectable with 2,4-dinitrophenyl (DNP)-labeled T4 bacteriophage; if any antibody was present, its concentration was below 2 µg/ml. Guinea pigs fed DNCB and then immunized with DNP on a foreign carrier, keyhole limpet hemocyanin (KLH), produced as much antibody 16 to 18 days later as did animals which had only been sensitized by intradermal injection of DNCB in phosphate-buffered saline and then immunized with DNP-KLH. The amount of antibody produced to DNP-KLH was measured at intervals. In the fed animals, the titer either remained approximately the same or increased with time. In contrast, the sera of sensitized guinea pigs contained a decreasing antibody titer when the first and last bleedings were compared. Isoelectric focusing was carried out with sera from DNP-KLH-immunized animals after the sera were first mixed with a radiolabeled hapten. The antibody-hapten profiles revealed that the DNCB-fed animals contained predominantly IgG2 in their serum by the time of their initial bleeding, whereas sensitized animals still contained a considerable proportion of more acidic antibodies having marked charge heterogeneity. Equilibrium dialysis measurements on purified antibody from fed animals indicated an average binding affinity for hapten equivalent to that of purified antibody from sensitized animals. Feeding of DNCB did not cause any obvious decrease in the number of lymphoid cells able to bind 125I-labeled guinea pig IgG. Lack of reduction of the average binding affinity coupled with the retention of antigen-binding lymphoid cells, permit the conclusion that feeding DNCB to guinea pigs under the conditions used did not induce an unresponsiveness or a deletion of those cells which have the potential to produce anti-DNP antibody. Thus, feeding of guinea pigs with DNCB in oil seems to induce a central tolerance at the T cell level without adversely affecting their B lymphocytes. The antibody response obtained by immunization with DNP conjugated to a foreign carrier is not only quantitatively equivalent to that obtained in control animals but qualitatively it appeared to be more mature with respect to charge at a relatively early time (16 to 18 days). Since the antibody levels in serum from fed animals increased or remained approximately constant with time instead of waning as occurred in the DNCB-sensitized animals, feeding or sensitization with DNCB may also have affected some cell or cell product important in the regulation of antibody production.