The study of the participation of T cells and the kinetic studies of cytokines by the RT-PCR method in the challenge phase of contact hypersensitivity reaction (CHR) were performed using skin tissues collected at regular intervals from mice sensitized with 2,4-dinitro-1-fluorobenzene (DNFB). The results obtained from these studies and the further analyses using anti-CD4 and CD8 antibodies showed that the revelation of phenotype of lymph node cells at the reaction site and T lymphocytes at the CHR site were found to play important roles. Furthermore, we examined cytokines formed at the CHR site on the mRNA level. Consequently, it was found that 24 h after the challenge IFN-gamma, IL-6 and IL-1 beta were expressed and 48 h after the challenge IL-2, IL-4 and TNF-alpha were expressed in addition to these cytokines. The results of the expression of IL-12 and those of the examination using IFN-gamma knock out mice suggested that the expression of IFN-gamma among several Th1 types of cytokine is especially important for the formation of CHR.