The technique of fluorescence redistribution after photobleaching was utilized to measure intercellular movement of low molecular weight fluorescent hydrophilic substances across the cell wall/membrane interface between contiguous soybean (Glycine max (L.) Merr. cv. Mandarin) root cells (SB-1 cell line) in tissue culture. Lipopolysaccharide (LPS) purified from Bradyrhizobium japonicum R110d, a Gram-negative bacterium that normally infects and induces nodulation in soybean roots in vivo, inhibits intercellular communication between the soybean cells in a dose-dependent manner. In contrast, LPS from noninfecting strains failed to yield the same effect. The inhibitory activity of the LPS was localized to the O-antigen region of the LPS.