A number of studies have reported that heavy metals are not only toxic for the organism but they may modulate immune responses. In the current study, the effect of 4-week administration of 200 ppm of PbAc(2), using different routes of administration (orally and intraperitoneal injection), on lymphatic organs was evaluated. In the thymus, the number of lymphocyte cells and the cellularity diminished significantly for both routes of treatment. Regarding the submaxillary lymph nodes, no significant variations took place. Cell-mediated immune response is commonly evaluated by cell proliferation assays. Mitogens are known to induce a vigorous proliferative response in lymphoid cells from mammals. An increase in the proliferation of T lymphocytes stimulated by concanavalin A and the proliferation of B lymphocytes stimulated with lipopolysaccharides was found in thymus for both routes of administration, whereas in the lymph nodes, there was a decrease in proliferation of T lymphocytes. Furthermore, lead administration by intraperitoneal route caused an effect on B and T lymphocyte subpopulations. Thus, there was an increase in B+ cells and a decrease in T+ cells. Regarding CD4+ and CD8+ T cells, there were only variations, concretely a drop in both subpopulations, in lymph nodes when lead was administered intraperitoneally. It is important to emphasize that an increase in apoptosis was found in this tissue. At the histological level, evident alterations were described in thymus both for the oral and for the intraperitoneal route. Therefore, it is possible to show that lead administered by both routes generated effects on an immunological level.