Morphometry has been performed on Morris 7288c hepatoma cells (HTC cells) grown in suspension, with special emphasis on lysosomes and related structures. The fractional volumes of the nucleus and mitochondria are 22.3 and 6.9%, respectively. Lysosomes have been identified by ferritin uptake over a 24-hr period; they represent 1.8 to 2.9% of the cellular volume. Other single membrane-bounded structures, including mainly endoplasmic reticulum, Golgi complex, and phagosomes represent 11.5% of the cellular volume. These data are consistent with estimations of the protein content of the corresponding subcellular structures inferred from cell fractionation experiments. HTC cells contain a total of 5.6 m 2 of membranes/cm 3 . Mitochondria account for 38% of this membrane area while endoplasmic reticulum, Golgi complex, and phagosomes represent 46%. Lysosomal membrane accounts for 4% and plasma membrane for 7.5%. Peroxidase labeling over 1 hr reveals a rather large phagosomal compartment: it amounts to 4.4% of the cellular volume and phagosomal membrane represents nearly 10% of all cellular membranes. It can be estimated that endocytosis results in the interiorization of an area equivalent to that of the plasma membrane in about 45 min. These data are discussed within the frame of current concepts of plasma membrane recycling. Our observations imply that about half of the plasma membrane is not immediately accessible to extracellular substances, since it resides temporarily inside the cell as phagosomal membrane. Moreover it can be estimated that in HTC cells the time needed for the transfer of a phagosome from the cell periphery to lysosomes and the reincorporation of its membrane in the plasmalemma must require between 0.5 and 4 hr.