125-I-labelled cesalin binds to KB cells and plasma membranes in a specific and saturable manner. At 0 degrees C the cesalin specifically bound to cells is completely displaceable by excess unlabelled cesalin, but at 37 degrees C only 50% can be removed after incubation for 2 h. The extent of binding to plasma membranes has the following characteristics: it is increased following treatment of membranes with cholate; treatment with trypsin has no effect on binding and neither is the bound 125-I-labelled cesalin removed following digestion with trypsin; binding is not inhibited by several carbohydrates but is decreased to about one half by concanavalin A. In addition it is found that some degradation of cesalin occurs with KB cells, the specific binding to which is not enhanced by chloroquine, a lysosomotropic agent. No loss of binding in cells is seen after 4 h exposure to cesalin, suggesting no reduction in the receptors by internalization. The data are consistent with a mechanism in which 125I-labelled cesalin is rapidly bound at 37 degrees C to a receptor on cell membranes through which the biological activity is effected. Slowly, some change in the bound cesalin occurs that prevents its complete displacement from the cells.