A major advance in high resolution EM immunoprobes has recently been achieved: The smallest gold particles easily seen directly in the TEM have been coupled to Fab fragments thus making them the smallest gold-antibody probe commercially available.The gold particle, NanogoldTM, is 1.4 nm in diameter with a very controlled size range, ± 10% (Fig. 1). This is in sharp contrast to other small gold preparations, such as Auroprobe One (Janssen Life Sciences) which actually ranges from 1-3 nm.The Fab conjugate (Fig. 2) has close to one gold particle per Fab fragment. This again is different from other gold-IgG probes that have 0.2-10 gold particles per IgG. Another difference is that the Nanogold-Fab conjugates are separate molecules in solution rather than the often extensive aggregation of other colloidal gold-IgG preparations. The problems of ratio of gold particles to antibody and aggregation in conventional colloidal conjugates were shown to be controllable by careful trial and error testing.