Human epidermal keratinocytes in culture form stratified colonies that retain many of the properties of intact epidermis. Mitosis is restricted to the basal layer, and cells that leave it undergo terminal differentiation, increasing in size and synthesizing a range of specialized molecules as they pass through the different cell layers. One useful marker for an early stage in the pathway of terminal differentiation is involucrin, a soluble protein precursor of the cross-linked envelope. Involucrin is synthesized after keratinocytes have left the basal layer and begun to enlarge, but some time before the onset of envelope cross-linking, which occurs only in the outermost cell layers. Small, involucrin-negative cells can be isolated and cultured in medium containing a low concentration of calcium ions, to prevent stratification, but not cell division and enlargement. Under these conditions, involucrin synthesis is initiated by some cells in the monolayer, indicating that attainment of a suprabasal position is not required for this step in terminal differentiation. Furthermore, if the level of calcium is raised to induce stratification, involucrin-positive cells are selectively expelled from the basal layer. This suggests that migration from the basal layer may be a consequence, not a cause, of terminal differentiation. Changes in cell-surface properties during terminal differentiation might explain the observed sorting out of involucrin-positive and -negative cells. We have therefore investigated lectin binding to different cell layers in natural and cultured human epidermis. Some lectins bind only to nonbasal keratinocytes, and the binding of peanut agglutinin, in particular, shows interesting correlations with involucrin expression, both in stratified colonies and in monolayers.