The mechanisms of developing infection in young, noncompromised individuals are not well understood. Colonization is a prerequisite for the development of infection. In humans, ligands serving bacterial colonization belong to common antigens. Consequently, a majority of individuals should be sensitive to infection at all times. We hypothesize that the temporal patterns of some infections and sensitivity to them are associated with sudden changes in the density and accessibility of common receptors. Endometrial samples from women having normal menstrual cycles were examined for histological location, receptor density, and in situ hybridization of Dr (decay-accelerating factor) ligands for Escherichia coli Dr fimbriae. Significant up-regulation and luminal expression of Dr ligands occurred during the secretory phase, whereas receptors were expressed in the basement membrane and in smaller quantities during the proliferative phase. This observation agrees with our hypotheses that some ligands recognized by bacterial adhesins change their compartmentalization and, most importantly, that they up-regulate expression at specific times.