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A one-step immunoassay for simultaneous detection of serum IgG and IgM antibodies to Borrelia burgdorferi has been developed. The assay is based on C1q, which binds to immune complexes containing IgG and/or IgM antibodies. Micro-beads pre-coated with antibodies to human C1q are mixed with human serum samples and fluorochrome-labelled B. burgdorferi flagellum antigen. In the presence of serum IgG and/or IgM antibodies to B. burgdorferi, fluorochrome-labelled antigen/antibody complexes are formed. These are then bound by serum C1q and are subsequently captured on the anti-C1q-coated beads. The sample is analysed on a flow cytometer and the presence of fluorescent beads is, thus, indicative of a positive test result. In the present study the sensitivity and specificity of the assay are compared to those of the indirect IDEIA B. burgdorferi IgG and the mu-chain capture IDEIA B. burgdorferi IgM ELISAs for separate determination of IgG and IgM. Detection using a flow cytometer can be performed without separation of the beads from the reaction mixture, which means that in practice, the method is carried out as a one-step assay and it is, thus, very suitable for automation. Other advantages of this kind of assay includes an antibody/antigen reaction which occurs in solution and the potential of using the method for the detection of antibodies against several antigens from the same or different infectious agents (multi-parameter screening).