Since 1967, major advances have been made in procedures to isolate and maintain keratinocytes in liquid medium. Human keratinocytes and those from several laboratory animal species may now be isolated from skin either by direct trypsinization of minces, from split-thickness skin following the separation of the epidermis from the dermis, or from the outgrowth from tissue explanted in liquid medium. Isolated keratinocytes display several distinct stages leading to terminal maturation. These include attachment, spreading, reassociation, multiplication, and maturation. Conditions under which each of these stages can be blocked are known and thus provide an opportunity to observe and characterize the biochemical and morphologic changes at each stage of maturation. Although keratinocytes in liquid simulate many of the typical and important characteristics observed in these cells in vivo, the conditions required to reproduce other important functions of keratinocytes have not yet been defined. These functions include the synthesis of basement membranes, lamellar bodies, and keratohyaline granules and the appropriate alignment of lipids and proteins in the completely keratinized cell.