Soluble concanavalin A (Con A), phytohemagglutinin-P (PHA), and pokeweed mitogen (PWM) stimulate DNA synthesis in cells from a variety of normal rabbit lymphoid organs. In lethally irradiated rabbits injected with thymocytes, these responses are virtually normal or, in some instances, increased. In thymus-deprived animals (thymectomy, lethal irradiation, bone marrow shielding) they are virtually abolished. We conclude that the substances tested are specific T cell mitogens in the rabbit, as in other species. The response to Con A is both of higher magnitude and wider distribution among various lymphoid organs than the response to either of the other two mitogens. Responses to PHA relative to Con A vary in different organs, being lowest in thymus and highest in popliteal lymph nodes and manually separated cells of the thymus-dependent area (TDA) of the appendix. PHA-responsive cells were found to have the same organ distribution as cells responding in the one-way mixed lymphocyte reaction (MLR). It is proposed that responsiveness to PHA develops only in differentiated, peripheral T cells and that the PHA/Con A response ratio provides a convenient index in the rabbit for estimating function in these cell populations. The appendix TDA was enriched in cells responsive to T cell mitogens and in the MLR relative to whole appendix as well as to the dome and follicle components of this organ. This finding confirms other evidence that this morphologic element of the gut-associated lymphoid tissue (GALT) is thymus-dependent. A small proportion of rabbit B lymphocytes, limited largely to peripheral lymph nodes, were shown to respond to endotoxic lipopolysaccharide. As is the case in other animals, PWM appears to be mitogenic for both T and B cells in the rabbit.