Between 10(3) and 10(4) auto-rosette-forming cells (RFC) per 10(6) lymphocytes are observed in lymphoid organs of normal rats in vitro. Counts are significantly higher in the thymus than in other organs. Contrary to what has been previously described in mice, auto-RFC are not inhibited by fresh normal rat serum. The data presented are compatible with the hypothesis that auto-RFC differ from the lymphocytes which recognize only alloerythrocytes according to histocompatibility differences. There is evidence which suggests that autorosette formation is linked to the expression of new determinants on ageing erythrocyte membranes.