High-angle annular dark-field (HAADF) STEM imaging is a sensitive and efficient technique for detecting immunogold labels. Larger (5-15 nm) gold labels can be distinguished clearly from the heavy metal stain on tissue sections, but for smaller (<5 nm) labels the distinction is less clear. It is not possible to differentiate between ultra-small (<1-3 nm) labels and stain, but the HAADF image shows sufficient contrast so specimens can remain unstained. On a TEM/STEM equipped with a LaB6 filament the smallest labels that are detectable are 1-2 nm. Efficient detection of ultra-small labels requires a field emission microscope.