Contact sensitizers are considered to lead to an immune response characteristic of Th1-type T helper cell activation, and interleukin-2 (IL-2) is a representative cytokine secreted by Th1 cells. In this study, contact sensitivity to 1-chloro-2,4-dinitrobenzene (DNCB) was evaluated using hapten-stimulated in vitro IL-2 production by the lymph node cells from mice contact-sensitized to DNCB. IL-2 activity in 48-h culture supernatants was measured by its ability to stimulate IL-2-dependent CTLL-2 cell growth using 3-(4,5-dimethylthiazol-xyl)-2,5-diphenyltetrazolium colorimetric assay, expressed as stimulation index. The results showed that DNCB-induced in vitro IL-2 production by the murine lymph node cells was hapten-specific. The peak of in vitro IL-2 production was observed on the 7th day after contact sensitization in mice. There was also a cross-reaction between DNCB and 2,4-dinitrobenzene sulfonic acid (DNBS) in in vitro IL-2 production. Strong dose-response relationships for DNCB-specific in vitro IL-2 production and for the cross-reaction to DNBS were further demonstrated. In addition, there was a good correlation between DNCB-induced in vitro IL-2 production and in vivo contact hypersensitivity as evaluated by mouse ear-swelling responses. Thus, the results suggest that hapten-stimulated in vitro IL-2 production by murine lymph node cells may be useful for predicting the contact-sensitizing capacity of chemicals.