Mesophyll cells were isolated from developing sink leaves (25 to 30 mm in length) of soybean, Glycine max (L.) Merr. cv. Will. Leaf strips were incubated for two h in a buffered medium containing osmoticum and 0.2% Pectolyase Y‐23. Gently stirring the leaf strips released from 7 to 16% of the total leaf mesophyll cells. Other pectinase enzymes, effective in releasing cells from mature source leaves (70 to 75 mm in length), did not release cells from sink leaves. Sink and source cell preparations were about 50 and 95% intact, respectively, based on the exclusion of Evans Blue dye. Intact cells could not be separated from broken cells on Ficoll or metrizamide density gradients. Total protein and catalase, glyceraldehyde‐3‐phosphate dehydrogenase, glycolate oxidase, phosphoenolpyruvate carboxylase, and ribulose 1,5‐bisphosphate carboxylase activities on a chlorophyll basis were about 50% lower in sink mesophyll cells than in sink leaf homogenates indicating that broken sink cells lost soluble protein to the medium. Source cells and source leaf homogenates had comparable amounts of protein and enzymatic activities. Enzymatic activities on a chlorophyll basis were similar in source and sink leaves with the exception of phosphenolpyruvate carboxylase, which was two times higher in sink leaves. This enzyme was also exceptionally low in source and sink cells being only 61 and 23%, respectively, of whole leaf activities. Sink cell rates of 14CO2 fixation were only 7% of source cell rates and sink cells did not show light‐dependent O2 evolution. Both cell preparations had photosystem II activiteis which were comparable to rates of 14CO2 fixation at satuarating light and CO2 concentration. It was concluded that the reduced photosynthetic rate of sink cells was limited by the low photochemical capacity rather than a limitation of Calvin cycle enzymes. Copyright © 1983, Wiley Blackwell. All rights reserved