An analysis of the relation of cell structures demonstrated by acid phosphatase staining to the lysosomes of DeDuve led to the following conclusions: 1) With acid phosphatase staining, structures derived from pinocytotic and reversed pinocytotic processes are visualized; 2) The acid phosphatase activity of various cells is related to the development and actual activity of the pinocytotic and reversed pinocytotic apparatus. The latter includes also secretion vacuoles; 3) The pinocytotic vacuoles migrate in the cell. They coalesce or fuse with other vacuoles and may become recognizable under the light microscope; 4) The vacuoles deriving from pinocytosis and reverse pinocytosis may be interpreted in their entirety as a cell organelle or system. For this system, "pinocytotic apparatus", as a descriptive term, is suggested.