Incubation of interphase murine mastocytoma cells with fluorescein-labeled wheat germ agglutinin (FITC-WGA) or with rhodamine-labeled concanavalin A (Rh-Con A) shows binding of the lectins to all the cells in a ring form distribution. Treatment of the cells with trypsin before addition of FITC-WGA, or with colchicine before addition of Rh-Con A, promoted rearrangement of the lectin binding sites into cap formation in 27-29% of the cells. Trypsin had no effect on cap formation by Rh-Con A, and colchicine had no effect on cap formation by FITC-WGA. When the same cells were pretreated consecutively with trypsin and colchicine followed by incubation with FITC-WGA and Rh-Con A, no increase in the percentage of cells bearing caps was observed. Moreover, the same cells showed co-capping with both lectins. Experiments with synchronous cultures of mastocytoma cells have indicated that capping induced by either lectin is maximal in the G2 phase and minimal in the G1 and S phases of the cell cycle. At the G2 phase 60% of the cells show cap formation either after trypsin-WGA treatment or after colchicine-Con A treatment. These data indicate that rearrangement of membrane receptors for Con-A and WGA in murine mastocytoma cells can occur only at certain phases of the cell cycle.