Allergic contact dermatitis (ACD) is mediated by hapten-specific CD8+ T cells and downregulated by CD4+ T cells. We have recently shown in a model of ACD to weak haptens that priming of IFNγ-producing CD8+ T cells and the development of skin inflammation could be obtained in mice deficient in CD4+ T cells. Here we show that IFNγ production by lymph node (LN) cells draining the site of skin sensitization of CD4+ T-cell-deficient mice is a marker of the sensitizing properties of weak haptens. LN cells from mice sensitized as in the classical local lymph node assay (LLNA) were recovered at day 5, then cultured for 20 hours in the presence of submitogenic doses of phytohemagglutinin, and finally tested for the production of IFNγ. Results show that: (i) production of INFγ by LN cells was induced by weak and moderate allergens in a dose-dependent fashion; (ii) the magnitude of IFNγ production paralleled the sensitizing properties of allergens allowing to classify them as moderate or weak haptens; (iii) chemicals without sensitizing properties were unable to stimulate IFNγ production by LN cells. Therefore, the IFNγ LLNA appears as a sensitive, specific, and robust assay to detect weak contact allergens.