The development of contact hypersensitivity (CHS) greatly depends on the allergenicity of the inducing agent. However, various cofactors are known to influence the outcome of the response as well. From this perspective, we have compared the effects of five different vehicles: acetone, ethanol, dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), and a 4 to 1 mixture of acetone and olive oil (AOO) on the cellular and humoral immune responses to epicutaneously applied oxazolone in female BALB/c mice. A single application of 0.2% oxazolone dissolved in acetone or ethanol induced stronger proliferative responses and higher lymph node cell numbers than the other three vehicles. Moreover, both vehicles led to higher numbers of oxazolone-specific Ab forming cells in the draining lymph nodes of sensitized animals. When the IgG2a/IgG1 ratios were determined to indicate the type of T helper cell involved, the highest values were obtained with AOO and lowest with DMF and DMSO, while acetone and ethanol were in between. Moreover, no correlation was found between oxazolone-specific antibody production and cellular responses, measured as [3H]thymidine incorporation of draining lymph node cells after sensitization and increased ear thickness after challenge. From this study it can be concluded that cellular and humoral responses in CHS to oxazolone are dissimilarly affected by the vehicles used.