1. Escherichia coli can accumulate 14C-labelled (alpha 1 leads to 4)-linked D-glucose oligomers up to maltoheptaose. Longer maltodextrins are not transported and are not utilized as carbon sources. 2. Maltodextrins too large to be transported are nevertheless bound by the outer envelope of intact E. coli. This binding is saturable (Kd for maltodecaose = 3-4 microM) and the binding sites are inducible by maltose. Each bacterium has approximately 30,000 sites when fully induced. 3. Using mutants devoid of various components of the maltose transport system, the high-affinity binding of maltodextrins by intact bacteria has been shown to be dependent on the presence of both lambda receptor (an outer membrane protein) and periplasmic maltose binding protein. 4. The same binding sites are accessible to both utilizable and non-utilizable maltodextrins. Maltodecapentaose is a competitive inhibitor of maltose transport (Ki 1.5-2.5 microM). 5. These results show that the periplasmic maltose binding protein is readily accessible to substrates of at least 2500 molecular weight. The inability to transport dextrins larger than maltoheptaose is, therefore, due to the inability of E. coli to transfer large substrates from the binding protein to the cytoplasm and not to lack of access through the outer membrane.