The mobilization of stored carbohydrates (sucrose and starch) and proteins during sucrose starvation was studied with sycamore (Acer pseudoplatanus L.) cells. When almost all the intracellular carbohydrate pools had disappeared, the cell protein content declined progressively whereas asparagine determined by either (13)C nuclear magnetic resonance or reversed phase high performance liquid chromatography increased steadily. After a long period of sucrose starvation, the most intense resonances in the (13)C nuclear magnetic resonance spectra were from citrate and asparagine. The total amounts of asparagine (expressed as nitrogen) and free amino acids that appeared after a long period of sucrose deprivation corresponded roughly to the total amount of protein (expressed as nitrogen), that disappeared within the same period of time. Addition of sucrose in the culture medium after a long period of sucrose starvation led to a disappearance of asparagine. These results suggest therefore that the presence of asparagine in plant cells in large excess should be considered as a good marker of protein utilization after a long period of sucrose starvation and is very likely related to stress.