natalizumab tysabri omalizumab xolair palivizumab synagis panitumumab vectibix tositumomab bexxar trastuzumab herceptin and other chimeric monoclonal antibodies the biological fluid to be analysed is e g serum plasma whole blood cerebrospinal fluid or synovial fluid sample preferably a serum sample in the competitive assay method according to the present invention cells or cell membrane preparations are bound to filter plate membranes cell lines expressing the required cell receptor cd52 and cell culture media are commercially available or can be prepared by methods known to persons skilled in the art cell membrane preparations can be prepared by homogenizing and subsequent centrifugation step by methods known to persons skilled in the art suitable filter plate membranes are commercially available and include e g acrowell filter plate membranes obtainable from pall life sciences a suitable detectable label for the purposes of the invention is a fluorescent label alternatively enzymatic and radioactive labels or magnetic particles may also be used if appropriate preferred fluorescent labels include all commonly used fluorescent labels such as europium eu labelling of the antibody can be carried out e g by labelling free amine groups the label is detected by using a label counter suitable for the detection of the label in question a calibration standard curve is provided by preparing calibration standards of the analyte antibody by measuring the signal and fitting the data to a standard curve preferably by using a suitable evaluation software in conclusion we have established a novel methodological concept for a sensitive and specific determination of a receptor bound antibody campath 1h alemtuzumab in biological samples such as serum more than ten fold improvement of lower limit.