The sedimentability of eight acid hydrolases in an homogenate from root tips of Zea mays (L.) varied between 13% (α-glucosidase) and 46% (β-N-acetylglucosaminidase) of the total activity. Between 20% and 30% of total activity of the majority of acid hydrolases examined was sedimentable.High specific activities of acid hydrolases (up to ten times the sedimentable or soluble activities) were associated with the cell walls, and considerable amounts of this activity (20-60%, depending on the enzyme) could only be released by cellulase treatment.Mitochondria-free membrane fractions isolated from the 8000 g, 25000 g and microsomal fractions contained acid hydrolase activities. The membranes could be separated into two bands on Ficoll gradients, but acid hydrolase activities were present in both bands.The 25000 g fraction was incubated with glucose-6-phosphate and lead nitrate to label the vesicles containing neutral phosphatase with lead phosphate. The labelled vesicles were separated from the remaining vesicles by density grandients. The majority of acid phosphatase, β-glucosidase, β-galactosidase and peroxidase activities were associated with the unlabelled fraction. Phosphodiesterase was present in both fractions. The peroxidase isozyme thought to be tonoplast-associated (Parish, 1975) was found in the unlabelled vesicles.It was concluded that sedimentable acid hydrolases are associated with a specific class or classes of membrane.More than 60% of the sedimentable acid hydrolase activities were strongly bound to membranes. A membrane model of lysosome action is proposed.The isozymes of acid phosphatase and β-galactosidase present in the cell wall, sedimentable fractions and the supernatant were examined.A number of tentative conclusions are drawn concerning "lysosomes" and acid hydrolases in plant cells.