An alternative to aqueous chemical fixation consists in immobilizing physically the specimen by freezing it as fast as possible without using any cryoprotectant. This Fast Freeze Fixation (FFF) followed by Freeze Substitution (FS) avoids osmotic artefacts due to the slow penetration of liquid chemical fixative. Associated with Immuno-Gold labeling (IGS), FFF-FS allows a more precise localization of antigens.Using the bioluminescent bacteria Vibrio harveyi, a comparison of IGS with an antibody directed against its luciferase (enzyme of the luminescent reaction) has been done after liquid chemical fixation versus FFFFS. This later technique, beside an expected improvement of the ultrastructure always shows a better preservation of antigenicity and a lower background. In the case of FFF-FS technique (Figure 3):–labeling in acrylic resin (LRWhite) is 2 to 4 fold more intense than in epoxy resin (Epon),–but the ultrastructure is always better in Epon.–but the ultrastructure is always better in Epon.–The addition of fixatives in the substitution medium, results in a decrease of labeling which is more important in the case of a mixture of fixatives than with osmium tetroxide alone; with one exception: the substitution with glutaraldehyde which produces a dramatic increase in the density of the labeling but also, at the same time, a swelling of the cells of about 30%.