The interaction of the pore-forming protein (porin) from the outer membrane of Yersinia pseudotuberculosis with the S- and R-forms of lipopolysaccharide (LPS) from this bacterium was studied. Analysis of the equilibrium binding of 125I-labeled S- and R-LPS, as well as the competitive inhibition of this reaction by lipid A, core oligosaccharide, and O-specific polysaccharide, suggests that there are binding sites on the porin molecule specific to these fragments of the LPS molecule. The binding of R-LPS occurs at independent sites of two classes with Ka 1.7 x 10(5) and 1.1 x 10(5) M-1. S-LPS interacts with porin with positive cooperation (h 1.6) and Ka 0.8 x 10(5) M-1. The number of binding sites was found to be nine and four R- and S-LPS, respectively. Molecular mechanisms of the interaction are discussed.