This experiment pursued the time course of contact hypersensitivity to 2,4-dinitro-1-fluorobenzene (DNFB) and histologic changes of the cutaneous reaction in mice. The contact hypersensitivity reached a maximum 4 days after sensitization (96.9 ± 6.7% vs. 22.7 ± 1.3% in control) and persisted for 3 weeks. The cutaneous hypersensitivity reaction showed peak reactivity at 24 hr after challenge (96.2 ± 4.7% vs. 11.5 ± 1.7% in control), and persisted up to 96 hr (13.2 ± 2.1%). Prime histologic changes observed in this experiment were the exocytosis of lymphoid cells and epidermal thickening which appeared at 20 hr after challenge. Edema, vasodilatation and increased mast cells were observed within the dermis at 4-8 hr. However, edema and vasodilatation disappeared gradually, but numbers of mast cell increased up to 96 hr. The dermal infiltrates were maximum at the 28-72 hr after challenge.