A new cationic colloidal gold complex has been developed for ultrastructural localization of cell surface anionic sites by transmission and scanning electron microscopy. The marker is prepared by labelling gold particles of suitable sizes (6 to 70 nm in diameter) with chitosan, a polymer of beta (1----4)-linked D-glucosamine. Using human red blood cells as a model, chitosan-gold complexes were shown to be specific for anionic sites and at pH 2 for sialic acid residues. The binding capacity of complexes of different sizes with carboxymethyl and phosphorylated celluloses was examined as a function of pH and ionic strength. The results indicated that these complexes can be used under acidic conditions as well as in physiological buffers. The complexes were further tested by transmission and scanning electron microscopy in detecting anionic sites on cells of various origins such as Escherichia coli, Lactobacillus maltaromicus, Lactobacillus reuteri, Saccharomyces cerevisiae, Saccharomyces rouxii, Schizosaccharomyces pombe, Fusarium oxysporum, Catharantus roseus.