A method was developed to separate protoplasmic contents from the cell wall causing polymer degradation. Alkali treatment (6 M NaOH at 37°C) was shown then to solubilise essentially all remaining (wall-bound) ³H-polymers. Gel-permeation chromatography (GPC) on Sepharose CL-4B was used to size-fractionate the isolated ³H-polymers. Alkali treatments, similar to those employed to extract wall-bound ³H-hemicellulose, were found to minimise aggregation of xylan and xyloglucan caused by sample storage and such treatments were applied to ³H-polymers from each of the cell compartments. GPC-fractionated ³H-polymers was Driselase-digested to diagnostic ³H-fragments; maximisation of [³H]xylan susceptibility to Driselase required an acid pre-treatment. The data presented illustrate the synthesis and subsequent secretion of ³H-polymers from the protoplasm to the cell wall, where they become integrated before subsequent degradation or sloughing to the culture medium. Protoplasmic [³H]xylan and [³H]xyloglucan were polymerised from 40 kDa to 2 MDa before secretion to the cell wall, where the hemicelluloses further increased in size in over 2 MDa. Within the medium, [³H]xylan and [³H]xyloglucan were approximately 1-2 MDa until 2 d after the onset of ³H-labelling, when they dramatically increased in size, probably due to phenolic coupling. The M_r values observed were much higher than reported for rose hemicellulose (Thompson and Fry, 1997). Attempts to ‘disaggregate’ the very high-M_r wall-bound ³H-hemicelluloses, using different GPC eluents, alkali and enzymic digestion, were unsuccessful. Alkali, however, was successful in ‘disaggregating’ the very high-M_r soluble extracellular [³H]xylan and [³H]xyloglucan, suggesting that alkali-labile bonds, more resistant than feruloyl esters, play a significant part in the extracellular cross-linking of xylan and xyloglucan. The in vivo formation of high-M_r complexes of soluble extracellular ³H-polysaccharides xyloglucan. The in vivo formation of high-M_r complexes of soluble extracellular ³H-polysaccharides (SEPs) was delayed by sinapic acid, chlorogenic acid and rutin but promoted by ferulic acid and tyrosine: these data support a role for phenolic groups in the extracellular cross-linking of hemicelluloses.