Small and medium size lymphocytes, obtained from lymph nodes of untreated rats, adhere to chicken red cells in the presence of heat-inactivated antiserum against the red cells. This occurs within incubation periods which do not exceed those required for agglutinin activity of the antiserum. There is little or no adherence of thymocytes to the red cells. Overt adherence requires 10–20 small lymphocytes/1 red cell in the reaction mixture. Prior treatment of small lymphocytes with antiserum does not affect their ability to adhere to the red cells. The adherence is slowed down at 0–4 °C. It is diminished by 4 × 10 −4 to 3.3 × 10 −3 m sodium cyanide but not by 4 × 10 −4 to 3.3 × 10 −3 m iodoacetate and EDTA. It is abolished by prior exposure of lymphocytes to 50 °C for 10 min. Most rat antisera against chicken red cells tested induced significant small lymphocyte-chicken red cell adherence. The degree of adherence varied, however, with the individual antiserum and individual lymph node from which lymphocytes were obtained.