The occurrence and function of the side chains occurring in the rhamnogalacturonan I domain of pectic poly- saccharides have been investigated during carrot cell development using monoclonal antibodies to defined epitopes of (1→4)-β-D-galactan and (1→5)-α-L-arabinan. Immunolocalization studies of carrot root apices indicated that cell walls in the central region of the meristem contained higher levels of (1→5)-α-arabinan than the cell walls of surrounding cells. In contrast (1→4)-β-galactan was absent from the cell walls of the central meristematic cells but appeared abundantly at a certain point during root cap cell differentiation and also appeared in cell walls of differentiating stele and cortical cells. This developmental pattern of epitope occurrence was also reflected in a suspension-cultured carrot cell line that can be induced to switch from proliferation to elongation by altered culture conditions. (1→4)-β-galactan occurred at a low level in cell walls of proliferating cells but accumulated rapidly in cell walls following induction, before any visible cell elongation, while (1→5)-α-arabinan was present in cell walls of proliferating cells but was absent from cell walls of elongated cells. Immunochemical assays of the cultured cells confirmed the early appearance of (1→4)-β-galactan during the switch from cell proliferation to cell elongation. Anion-exchange chromatography confirmed that (1→4)-β-galactan was attached to acidic pectic domains and also indicated that it was separate from a distinct homogalacturonan-rich component. These results indicate that the neutral components of pectic polysaccharides may have important roles in plant cell development.