We identified and isolated a monoclonal antibody (MAb 3G2) raised against extracellular proteins from microcluster cells of orchard grass (Dactylis glomerata L.) embryogenic suspension culture. MAb 3G2 recognized with high specificity an antigen ionically bound within the primary cell wall and in the culture medium of microcluster cells. Two-dimensional polyacrylamide gel analysis and blotting of proteins on PVDF membrane showed that MAb 3G2 detected a single polypeptide of apparent molecular mass of 48 kDa and an isoelectric point (pI) of 5.2, designated EP48. A transient expression during somatic embryogenesis was observed for EP48. Indirect immunofluorescence showed that this protein highly accumulated in the cell walls of some single cells, microclusters and partly in proembryogenic masses (PEMs), but not in globular embryos of the embryogenic cell line and microclusters from the non-embryogenic cell line. Signal intensity varied between individual cells of the same population and in successive stages of somatic embryo development. Screening of several D. glomerata L. embryogenic and non-embryogenic cell lines with MAb 3G2 indicated the presence of ECP48 in only embryogenic suspension cultures at early stages of embryo development long before morphological changes have taken place and thus it could serve as an early marker for embryogenic potential in D. glomerata L. suspension cultures.