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Two-site enzyme-linked immunosorbent assays (ELISA) have been established for the specific and sensitive determination of two membrane proteins of the small synaptic vesicles (SSV), namely: peripheral synapsin I and integral synaptophysin. The ELISA used highly specific capture monoclonal antibodies (mAB) and polyclonal antibodies (pAB) as detectors. For synapsin I, the mAB were newly generated, whereas for synaptophysin, the commercially available mAB SY38 was applied. In order to calibrate the ELISA and to raise pAB, both proteins were purified in the mg-range. Synapsin I was purified by conventional means from human and porcine brain and synaptophysin was purified by immunoaffinity chromatography from porcine brain. Using the ELISA, neither synapsin I nor synaptophysin could be determined in serum or cerebrospinal fluid (CSF) from healthy donors or patients suffering various neurological disorders or pheochromocytomas. For this reason, the degradation of both proteins in serum and CSF was investigated. With the exception of synaptophysin measured in serum, both proteins exhibited fast rates of degradation. Despite the negative results in human body fluids, the two ELISA are appropriate for the quantification of these membrane proteins in neuronal or neuroendocrine cell extracts or preparations of SSV.