The aim of this work was to discover which compound(s) cross the amyloplast envelope to supply the carbon for starch synthesis in grains of Triticum aestivum L. Amyloplasts were isolated, on a continuous gradient of Nycodenz, from lysates of protoplasts of endosperm of developing grains, and then incubated in solutions of (14)C-labelled: glucose, glucose 1-phosphate, glucose 6-phosphate, fructose 6-phosphate, fructose-1,6-bisphosphate, dihydroxyacetone phosphate and glycerol 3-phosphate. Only glucose 1-phosphate gave appreciable labelling of starch that was dependent upon the integrity of the amyloplasts. Incorporation into starch was linear with respect to time for 2 h. At the end of the incubations, 98% of the (14)C in the soluble fraction of the incubation mixture was recovered as [(14)C]glucose 1-phosphate. Thus it is unlikely that the added [(14)C glucose 1-phosphate was extensively metabolized prior to uptake by the amyloplasts. It is argued that the behaviour of the isolated amyloplasts, and previously published data on the labelling of starch by [(13)C]glucose, are consistent with the view that in wheat grains it is a C-6, not a C-3, compound that enters the amyloplast to provide the carbon for starch synthesis.