It is suggested that red cells have an immune function as well as a respiratory one. Adherence of red cells to antigen-antibody-complement complexes (red-cell immune adherence) has been observed in primates and in rabbits and may represent a basic immune phenomenon that is common to at least all mammals. It is estimated that 95% of C3b receptors in the human circulation are located on red cells and that an antigen-antibody-complement complex has a 500-1000 times greater chance of being removed from the circulation by a red cell than by a white cell. That red cells also adhere to autologous thymocytes and T cells suggests that they may act as intermediaries bringing antigens and T cells together.