The evolution and dynamics of lysosomes in keratinocytes conform to the general lysosome concept of DeDuve and Wattiaux. Increased lysosome formation can be induced experimentally. Protein uptake by keratinocytes stimulates the synthesis of lysosomal enzymes and heterophagosomes (containing the ingested protein) are transformed into heterolysosomes which then perform digestive functions. The Golgi apparatus represents a source of primary lysosomes which supply acid phosphatase to the phagosome population. On the other hand, injury to the epidermis enhances the formation of autolysosomes within keratinocytes. Damaged cytoplasmic components are* segregated into acid phosphatase containing vacuoles where they become subject to autodigestion.