Movements of the receptors of concanavalin A on various parts of the surfaces of substrate-attached cells were compared. Cultured mouse embryo cells of several types were used: epithelial kidney cells and normal and transformed fibroblasts. Initial distribution of receptors was random on the cells of all types. Binding of concanavalin A induced patching of its receptors on all the cell parts. In contrast, directional centripetal movement of receptors was observed only on the surface of certain cell parts, namely, only the surface of peripheral lamellar cytoplasm was cleared of the receptors.Clearing was always initiated in the zone of lamellar cytoplasm located near active cell edges. In epithelial sheets, clearing was not observed on the surface of central cells that had no lamellar cytoplasm. Concanavalin A receptors on the cleared areas of cell surface were gradually restored after the end of incubation. It is suggested that anchoring of the patches of membrane receptors by cortical microfilaments is possible only on the surface of pseudopods and of lamellar cytoplasm but not on the surface of other cell parts. Besides receptor movements, this hypothesis may also explain differences in the adhesive properties of various parts ol the cell surface.The external edge of substrate-spread cultured cells is usually divided into discrete zones of two types: active zones that continuously protrude pseudopods and stable zones that do not form pseudopods. Specialized peripheral structure, lamellar cytoplasm or lamelloplasm, is formed in association with active cell edges (1,2). Distinctive characteristics of lamellar cytoplasm, as opposed to the more central endoplasm, are as follows: (a) lamelloplasm is delimited by an active edge; the entire external edge of lamelloplasm is not active, but usually all the active zones of the edge delimit lamelloplasm; stable zones may delimit either lamelloplasm or endoplasm. (b) Lamelloplasm contains no vesicular organelles visible by phase contrast microscopy. (c) The lower surface of lamelloplasm has numerous sites of attachment to the substrate. (d) The upper surface of the zone near active edges, and, possibly, of the whole lamelloplasm, is adhesive for inert particles (3, 4). Defective formation of lamellar cytoplasm is a characteristic feature of transformed fibroblasts (1, 2).In this paper we compare the movements of concanavalin A (Con A) receptors on the surface of lamelloplasm with those on the other cell parts. Con A and other ligands crosslinking surface receptors induce redistribution of these receptors (5-10). Results obtained with substrate-attached fibroblasts indicate that patches of the receptors may move centripetally from the active cell edges (7-9). Our purpose was to compare receptor movements on the cells in two states: with lamelloplasm and without it. Epithelial cells are especially convenient for such comparison: these cells grow as coherent sheets in which lamelloplasm and active cell edges are formed only by marginal cells but not by the central ce...