Serum parameters as indicators for the efficacy of therapeutic drugs are currently in the focus of intensive research. The induction of certain cytokines (or cytokine patterns) is known to be related to the status of the immune response e.g. in regulating the T(H)1/T(H)2 balance. Regarding their potential value as surrogate parameters in clinical trials and subsequently for the assignment of treatment efficacy, the accurate and reliable determination of cytokines in patient serum is mandatory. Because serum samples are precious and limited, test methods-like the xMAP multiplex technology-that allow for the simultaneous determination of a variety of cytokines from only a small sample aliquot, can offer great advantages.We here have compared multiplex kits from three different manufactures and found striking differences upon standardizing using WHO standards for selected cytokines. We therefore extended our xMAP multiplex measurements investigations to an ex-vivo situation by testing serum samples and found that the cytokine amounts measured was critically influenced by the actual kit used. The presented data indicate that statements regarding the quantitive determination of cytokines-and therefore their use as biomarkers-in serum samples have to be interpreted with caution.