Movement of aluminium into excised root tips of sainfoin follows a normal curve which suggests little or no uptake into non-free space. Washing in water removes very little aluminium from the roots but washing in dilute buffer solution removes a considerable exchangeable fraction. The pattern of phosphate incorporation in roots pretreated with aluminium is different from that obtained with untreated roots in several respects. Both percentage esterification and the proportions of the esterified fractions differ suggesting both a Donnan free space and a metabolic interference. The small amount of aluminium removed by aqueous washing suggests a minimum of precipitation with aluminium either external to the root, or in the water-free space of the cortex. The importance of this aluminium-phosphate effect as a component of aluminium toxicity, its specific sites and rates of reaction, require further experimental assessment.