A substantial proportion of mouse spleen and thymus cells form spontaneous rosettes with syngeneic and allogeneic erythrocytes. Rosette formation is independent of the metabolic state of the cell: Inhibitors of ATP formation (NaCN, NaN 3) and of cell motility (cytochalasin B, colchicine, vinblastine) have little or no effect. Erythrocytes can be bound by living or dead lymphocytes. The stability of rosettes against high temperature and shearing forces is weaker than that of immune rosettes. The increase or decrease of electrostatic repulsion between cells will enhance or reduce rosette formation. Membrane proteins are involved in cell-to-cell contacts, since several proteases destroy relevant structures on the cell surfaces of lymphocytes, erythrocytes, or both.