Antigenic competition in contact sensitivity was found to occur between DNFB and PCl and also between some other pairs of non-cross reacting sensitizers. When mice were painted with DNFB 0-7 days before the sensitization with PCl, the responsiveness to PCl was suppressed completely, whereas when DNFB was painted after sensitization with PCl, the responsiveness to PCl was affected. The prior application of DNFB resulted in the reduction of DNA synthesis in the draining lymph nodes of PCl-sensitization. This indicated that the suppression by painting with DNFB was not attributable to the inactivation of PCl-specific effector T cells which had already been generated by the sensitization with PCl, but to the inhibition of the generation of PCl-specific effector T cells. The antigenic competition occurred even when two contact sensitizing agents were applied to different lymphoid regions of mice. The suppressive effect of DNFB on the PCl-sensitization was observed also in the mice which had been made tolerant to DNFB. These results seem to imply that the antigenic competition in the present experimental system is the phenomenon which may not be mediated by effector T cells but rather by other cells, possibly suppressor cells.