The root of Arabidopsis thaliana (L.) Heynh. undergoes dramatic morphological changes during secondary thickening. We have used monoclonal antibodies that recognize two cell surface arabinogalactan protein epitopes (AGPs) and a pectic polysaccharide to document cell surface changes during the process of secondary thickening. These antibodies recognize cells in various stages of differentiation. An AGP epitope recognized by JIM 14 is expressed at the plasma membrane of most tells in the root, but is most highly expressed in mature sieve tube elements, JIM 13 identifies an epitope that is expressed in a discrete ring of cells in the periderm and during xylem vessel element differentiation, disappearing as lignificntion of the vessel elements proceeds, These antibodies reveal a biochemical differentiation at the surface of cells which mirrors obvious morphological differentiation events. Since AGPs have been shown previously to have a dramatic effect on the developmental capabilities of cells it is possible that the epitopes recognized by these antibodies might have an important developmental role during differentiation.