Despite a number of recent studies on appendix its function appears to remain unknown. The present studies were undertaken in order to extend and confirm the previous studies concerning the role of appendix in immune response. An early hemagglutinin response of mercaptoethanol sensitive antibody(IgM antibody) in rabbit injected intravenously(i.v.) with 200mcg of bovine gamma globulin(BGG) was abolished by lethal whole body irradiation(900 r), but preserved in animals whose appendix and bone marrow were shielded during irradiation. Late formation of mercaptoethanol resistant antibody(IgG antibody) and the development of memory in bone marrow shielded animals were not affected by irradiation of the appendix. Formation of either IgM or IgG antibody to sheep red blood cells(SRBC) injected i.v. as determined by direct plaque forming cell(DPFC) technique in spleen were effectively abolished by appendectomy, thymectomy, or both followed by irradiation. When bone marrow was shielded in combination with autologous appendix reconstitution, DPFC response was about 5 times greater than the sum of two. Lysed appendix cells failed to restore the response. Lethally irradiated rabbits restored with combination of autologous appendix and thymus cells showed DPFC responses which were essentially normal. Three pools of appendix were obtained by manual separation technique and were stimulated with soluble concanavalin A(Con A), phytohemagglutinin-P(PHA) and pokeweed mitogen(PWM). Rabbit appendix cells responded to Con A, PHA and PWM. Cells of thymus dependent area(TDA) of the appendix were relatively enriched in their response to T cell mitogens compared to dome and follicle cells. The PHA/Con A responsive ratio of appenix TDA subpopulation was high, indicating that Con A responsive cells have a wider distribution among appendix. This finding showed that interfollicular area of the appendix is thymus-dependent. The present studies confirmed other evidence that the rabbit appendix cells itself are unable to form antibody and T lymphocytes in appendix TDA may be heterogenous, and that the appendix cells are synergistic with either bone marrow or thymus cells in the early hemagglutinin on splenic antibody response to BGG or SRBC.