This chapter reviews methods where bacteria and bacterial adhesins can be applied as tools for defining specific cell lineages and for characterizing eukaryotic receptor epitopes, primarily based on experiences with Helicobacter pylori, Streptococcus pyogenes, Escherichia coli, and Actinomyces spp. In situ screening of histological sections has been used to reveal the cell specific distribution of bacterial receptors among different cell populations in target tissues using histochemical and immunohistochemical stainings, in situ DNA hybridization, or electron microscopy. Direct binding of fluorochrome conjugated E. coli to frozen sections of human kidney has been described. A powerful adaptation of this technique developed in the chapter utilizes the bacteria themselves as probes to study the distribution of receptor epitopes in histological sections of potential target tissues. The advantage with this model is that it allows direct evaluation of the cellular distribution of receptor molecules, and that it can be used to characterize the receptor directly on the tissue by performing double labeling, biochemical modifications, and inhibition assays.