Cell surface protein were found to play a role in the sugar-specific molecular mechanism by which bacteria adhere to mammalian cells. We have demonstrated that at least three different types of lectin-like proteins mediate the mannose-sensitive adherence of gram negative bacteria to epithelial cells. One group of such lectins was shown in our study to be associated with the bacterial flagellum. Flagella isolated from Escherichia coli 7343 and Serratia marcescens 8347 exhibited mannose-sensitive agglutination of yeast cells; however, the flagella of the two bacteria differ in the molecular structure of their protein subunits. Another class of lectins comprises the bacterial fimbriae (also known as type 1 pili), which were previously shown to facilitate the mannose-sensitive adherence of various bacteria to mammalian cells. Fimbriae isolated from E. coli 346 were reversibly dissociated by saturated guanidine hydrochloride to their protein subunits. The dissociated subunits retained in part their mannose-binding ability, and were reassembled into fimbriae-like structures by removal of the denaturant under specific conditions. Mannose-sensitive yeast agglutinating activity of E. coli 2699, as well as of its isolated outer membranes devoid of fimbriae or flagella, was abolished by pretreatment with trypsin. It is therefore believed that the mannose-sensitive adherence of these bacteria is mediated also by lectin-like proteins associated directly with the outer membrane.