Maize root homogenates were prepared and centrifuged to sediment the mitochondria. The supernatants (6 KS) and pellets (6 KP) were collected and fractionated on linear sucrose density gradients. The distribution of carboxypeptidase was similar to that of α-mannosidase (a soluble vacuolar enzyme) and glucose-6-phosphate dehydrogenase (a soluble cytoplasmic enzyme). Carboxypeptidase was therefore a soluble in maize root cells and cannot be used as a marker for the tonoplast.