The immunoreactivity of the VLP surface glycoproteins was verified by staining purified unfixed VLPs by the immunogold labeling technique using NiV-specific mouse antiserum and 6 nm colloidal gold particle-conjugated goat anti-mouse secondary antibody (Jackson ImmunoResearch Laboratories Inc). The particles were viewed by EM after negative staining. The use of unfixed particles assured that only the surface-exposed antigens would be reactive. Numerous VLPs with the gold particles decorating their surface were seen; Figure 2E shows two such VLPs.