An in vitro exposure system was developed to enable simultaneous exposure of primary cells or cell lines to defined concentrations of volatile organic compounds (VOC) without the necessity of a constant-flow exposure system. Toluene was used as model VOC and administered via the gas phase. CD3/CD28-stimulated human peripheral blood mononuclear cells (PBMC) were used as indicator cells. Vitality/proliferation of PBMC was tested using the MTT assay and their functional reactivity using cytokine ELISA for interferon-γ (IFN-γ), interleukin-4 (IL-4), IL-13, and tumor-necrosis-factor-α (TNF-α). Chemical analysis using headspace gas chromatography confirmed that this new method guaranties reproducible VOC exposure ( R 2 = 0.995 for the correlation between external toluene concentration and toluene in the cell culture). While cytotoxic effects were not observed, dose-dependent toluene effects on functional reactivity of PBMC were found. The secretion of IFN-γ, IL-4, and IL-13 was inhibited at concentrations of 72.5 g/m 3 and above, whereas the TNF-α production was increased. Since the presented in vitro model ensures toluene exposure in concentrations comparable to the real situation, and allows the investigation of dose-dependent immunomodulatory toluene effects in concentrations without cytotoxicity, this method first described here is introduced as useful tool in analysis of VOC-triggered effects on immune cells.