The lambda receptor is a peptidoglycan-associated integral protein that spans the outer membrane. Beside its function in phage lambda adsorption it participates in transport. The latter function can be summarized as follows: 1) Receptor allows the nonspecific permeation of small molecules other than maltose and maltodextrins (in close analogy to a molecular sieve). Here the only criterion for selectivity is size and it has the properties of an unspecific pore. In this respect, it is similar to the outer membrane proteins Ia, Ib, and Ic, the porins. 2) It is a binding protein for maltodextrins. Binding affinity is low but increases by a factor of 500 as the chain length of the maltodextrins increases. In contrast, the affinity of the periplasmic maltose-binding protein for maltose and maltodextrins is similarly high (in the microM range). 3) In the in vitro system of liposomes, the lambda receptor facilitates specifically the diffusion of maltodextrins that exceed the size limit given by its porin function. This clearly demonstrates that the lambda receptor alone is able to specifically overcome the permeability barrier of the outer membrane for maltodextrins. 4) From the genetic and kinetic analysis of maltose and maltodextrin transport, it can be concluded that the lambda receptor interacts with the periplasmic maltose-binding protein. 5) Electron microscopic studies indicate a location for the maltose-binding protein in the outer cell envelope. This location is dependent on the presence of the lambda receptor.