When bound to cell surfaces, certain lectins such as concanavalin A induce a drop in the average diffusion coefficients (D) of a number of cell surface molecules. To find whether such anchorage modulation occurs naturally, D of surface antigens on different cell and tissue types were measured by fluorescence photobleaching recovery. Values for cells of the same tissue origin under different conditions of growth and association - in tissues, in small aggregates, and as isolated cells - varied by less than twofold when polyspecific monovalent antibodies to cell surface antigens were used, a range much less than the sixfold decrease in D observed after lectin-induced anchorage modulation. Thus, if reversible modulation of the diffusion rate is used naturally as a means of cell signaling, it must involve only a few kinds of surface receptors not detected by the antibodies used in this study. In certain tissues, however, a significant proportion of cells showed no apparent receptor mobility. This "all or none" modulation of lateral diffusion may reflect relatively long-lasting alterations in the states of a single cell type or differentiation among the cells of the particular tissue.