Passive transfer of tritiated thymidine-labeled lymphoid cells sensitized to the simple chemical DNFB into homologous guinea pigs resulted in positive contact skin reactions 24 hours after skin testing with DNFB. Labeled sensitized cells were found to accumulate at these sites, whereas, labeled nonsensitized lymphoid cells did not appear non-specifically in contact skin reaction sites. The labeled cells were small and large lymphocytes and immature cells of the lymphoid series. The maximum reactions were obtained at 24 hours, with an average of 3.4 per cent of the infiltrating mononuclear cells showing a label. At 48 hours, the macro- and microscopic reactions were similar to the 24 hour reactions but diminished in intensity, and the number of labeled cells in the infiltrates had decreased to 1 per cent of the total infiltrating mononuclear cells. (1/4) to (1/3) of the labeled cells were found within the epidermis in the test skin sites. These data have indicated that contact sensitivity, like tuberculin sensitivity, required the sensitized cell to initiate the skin reaction and that the majority of the cellular infiltrate was the result of non-specific host response to injury.