The starch polysaccharides amylose and amylopectin are not utilized by Escherichia coli, but are bound by the bacteria. The following evidence supports the view that the outer membrane lambda receptor protein, a component of the maltose/ maltodextrin transport system is responsible for the binding. (i) Amylose and amylopectin both inhibit the transport of maltose into E. coli. (ii) Both polysaccharides prevent binding of non-utilizable maltodextrins by the intact bacterium, a process previously shown to be dependent on components of the maltose transport system (T. Ferenci, Eur. J. Biochem., in press). (iii) A fluorescent amylopectin derivative, O-(fluoresceinyl thiocarbamoyl)-amylopectin, has been synthesized and shown to bind to E. coli in a reversible, saturable manner. Binding of O-(fluoresceinyl thiocarbamoyl)-amylopectin is absent in mutants lacking the lambda receptor, but mutations in any of the other components of the maltose transport system do not affect binding as long as lambda receptor is present. (iv) Using the inhibition of lambda receptor-dependent O-(fluoresceinyl thiocarbamoyl)-amylopectin binding as an assay, the affinities of the lambda receptor for maltodextrins and other sugars have been estimated. The affinity for dextrins increases with increasing degree of polymerization (K(d) for maltose, 14 mM; for maltotetraose, 0.3 mM; for maltodecaose, 0.075 mM). Maltose and some other di- and trisaccharides are inhibitory to amylopectin binding, but only at concentrations above 1 mM.