The interface between the host cell and the microsymbiont is an important zone for development and differentiation during successive stages of the rhizobium-legume symbiosis. Legume root nodule extensins, otherwise known as arabinogalactanprotein-extensin (AGPE) are abundant components of the infection thread matrix. We have characterized the origin and distribution of these glycoproteins at the symbiotic interface of root nodules of symbiotically defective mutants of pea (Pisum sativum L.) using immunogold localization with MAC265 an anti-AGPE monoclonal antibody. For mutants with defective growth of infection threads, the AGPE epitope was abundant in the extracellular matrix surrounding infected host cells in the central infected tissue of the nodule as well as being present in the lumen of Rhizobium-induced infection threads. This suggests a mis-targetting of AGPE as a consequence of abnormal growth of the infection threads. Furthermore, mutants in gene sym33 showed reduced labelling with MAC265, and in infection threads and droplets label was completely absent, a phenomenon not observed in wild-type nodules. This suggests an alteration in the composition of the infection thread matrix for sym33 mutants which may be correlated with the absence of endocytosis of rhizobia into the host cytoplasm.