Incubating isolated soybean leaf mesophyO ceOs with glyoxylate increased the rates of CO2 fixation by as much as 150%. In order to cause this stimulation, the glyoxylate must be presented to the cels before the NaHCO. (13,20). Photosynthesis was assayed in stoppered 10-ml Fernbach flasks in a reaction mixture that contained 0.3 M sorbitol, 50 mm Tris-HCl (pH 7.8), 2 mm NaNO3, 2 mm EDTA, 1 mm MnCl2, 1 mM MgCl2, 0.5 mm K2HPO4, and 2 mm DTT (added fresh daily).The isolated soybean leaf cells, containing 5-30 jig Chl and suspended in the same solution, were generally added to previously equilibrated flasks in a 25 C waterbath and illuminated for 15 s to 20 min in the absence of added NaHCO3. Next the NaH'4CO3 was added and the illumination continued for an additional 5 min before the reaction was stopped by adding 0.1 ml 3 N HCI. The amount of acid-stable radioactivity was determined by liquid scintillation counting of 0.1-or 0.2-ml aliquots of the acidified reaction mixture.All reactions were in equilibrium with air unless otherwise indicated. In some experiments the reaction vessels were equilibrated with C02-free gases by blowing the gas vigorously against the liquid surface for 10 min before the cells were added. The activity of the RuBP carboxylase in the cells was assayed by first making the membrane more permeable with toluene (7) and then adding the carboxylation substrates RuBP and NaH14CO3. The cells were preilluminated and assayed in the same solution that was used in the photosynthetic studies except that the MgC12 concentration was increased to 25 mm (1). After the cells were illuminated at 25 C for 5 min, 25-,lI samples of the cell mixture were transferred to the reaction flasks. Thirty s later the reaction was terminated by adding 0.1 ml 3 N HCI. The reaction flasks contained 50 pl of a 4:1 (v/v) ethanol to toluene solution, 0.35 mM RuBP, the indicated amount of NaH'4CO3, (2)(3)(4)uCi/,umol)