Secondary antibody coated colloidal gold probes for use in the electron microscopical immunogold (silver) staining technique. Seven separate specificities of secondary antibodies, all affinity-purified on the corresponding antigen, are available. The anti-mouse IgG and IgM antibodies are cross-adsorbed to eliminate cross-reaction. A mixed anti-mouse IgG+IgM specificity is also available, which recognizes both IgM and IgG. Anti-rabbit IgG and anti-mouse IgG are available, coupled to 5, 10, 15 and 30nm gold particles. Anti-mouse IgM is available, coupled to 5, 10 and 30nm gold particles. Anti-mouse IgG+IgM, anti-human IgG and anti-rat IgG are available, coupled to 5 and 10nm gold particles. Anti-goat IgG is available, coupled to 5nm gold particles. The different sizes are not overlapping. The AuroProbe EM reagents are manufactured and purified according to a novel large-scale production method. This method eliminates virtually all free antibodies and aggregates of gold particles and generates very narrow and constant size distributions. The particle size distribution and batch number are given on a separate sheet. Information on the relative paucity of clumps is also provided. The absence of clumps is essential for a high quality gold probe. AuroProbe EM reagents are calibrated to have an OD at 520nm of 2.5 for the 5 and 10nm sizes, 3.5 for the 15nm and 7.0 for the 30nm sizes. They are suspended in 20mM Tris buffered saline, pH8.2, supplemented with 0.1% BSA and 0.05% sodium azide: 0.1% BSA-Tris.