A relatively high concentration of 2,4‐dichlorophenoxyacetic acid (45 μM) in solid culture medium stimulated the formation and secretion of mucilage polysaccharides by callus tissues ofArabidopsis thalianaL. Heynh. (line Estland). The mucilage was composed of at least two polysaccharides as revealed by gel chromatography on Sepharose 4B: the major component (87%) eluted in the void volume (molecular weight 2 × 106or greater) and the minor component (13%) eluted in the molecular weight range from 2 × 104to 4 × 105. Both polysaccharide components contained small amounts of uronic acids. The major polysaccharide consisted mostly of galactose (49%), arabinose (28%) and fucose (10%), whereas the minor one consisted of galactose (44%), xylose (18%), arabinose (14%) and rhamnose (14%). One of the components of the secreted mucilage seems to be an arabinogal