The synthesis of 2-carboxy-D-arabinitol-1-phosphate (CA1P), the naturally occurring inhibitor of ribulose-1,5-bisphosphate carboxylase/oxygenase, was studied in leaves of the French bean Phaseolus vulgaris, L. Leaves were supplied with air containing ¹⁴CO₂ in the light then the plants were transferred to normal air in the light or in the dark. Leaf samples were frozen in liquid nitrogen, ground to a powder and extracted with acid. Lipids, pigments and cations were removed from the extract and CA1P and 2-carboxy-D-arabinitol (CA) recovered by anion exchange chromatography. The CA1P was further purified by its specific binding to purified ribulose-1,5-bisphosphate carboxylase/ oxygenase. CA and CA1P were identified by chromatographic properties and n.m.r. spectra. When plants were kept for 15 h in darkness after exposure to ¹⁴CO₂, up to 2.2 % and 5.5 % of the radioactivity in the extracts was present in CA1P and CA, respectively. The most radioactivity appeared in these compounds when photosynthesis from ¹⁴CO₂ took place at low photosynthetic photon flux density (PPFD). Under such conditions, radioactivity was detected in CA1P after only 10 min. During subsequent exposure to normal air (¹²CO₂) at low PPFD the amount of radioactivity in CA1P remained almost constant for 6 h; in darkness the rate of incorporation of radioactivity into CA1P reached a maximum after 2 h and the radioactivity was still increasing 6 h later. At low PPFD, the amount of CA1P in the leaves reached a maximum after 2 h. In darkness, the amount of CA1P began to increase rapidly after a lag of almost 1 h, well ahead of the increase in radioactivity in CA1P.