Cells suppressing red blood cell (RBC) autoantibody responses were found to exert their effect in Lyt-2+depleted recipients. Mice injected with monoclonal antibodies to Lyt-2 became deficient in Lyt-2+cells as judged by indirect immunofluorescence. These mice and normal recipients were given either rat RBC primed spleen cells (suppressor cells) or normal spleen cells and challenged with rat RBC. The autoantibody response of both Lyt-2+depleted and normal recipients was suppressed as compared with that of mice given normal spleen cells. It is suggested that an Lyt-1+cell may be involved in the expression of suppression.