Some kinetic properties of partially purified phosphoenolpyruvate carboxylase (PEPCase) from guard-cell and mesophyll-cell protoplasts of Commelina communis are described. The PEPCase activity inherent to each cell type was determined and the apparent K m (phosphoenolpyruvate) and K i (malate) were compared. Malate sensitivity was much higher (K i malate 0.4 mol m−3) in the extract of guard-cell protoplasts than in that of mesophyllcell protoplasts (K i malate 4.2 mol m−3). The stimulation of activity by glucose-6-phosphate in the presence of malate (‘deinhibition’) was also investigated in extracts from both cell types and was found to be similar to previously reported results with epidermal tissue. The effect of contamination of an extract of guard-cell protoplasts with mesophyll-cell protoplasts was measured in the presence and absence of malate. It was found that a small amount to mesophyll-cell contaminant appears to desensitize the malate inhibition of PEPCase from guard-cell protoplasts. It is concluded that experiments which use epidermal tissue to study guardcell PEPCase may give misleading information as a consequence of mesophyll contamination.