The pathology of the spleen, lymph nodes and thymus of C57L/J mice, infected intraperitoneally with 20 and 100 cysts of Echinococcus multilocularis is described at 2, 4, 8 and 12 weeks postinfection (p.i.). For the first 8 weeks, growth of the larval cyst mass (LCM) was slow and blastogenesis in T-dependent areas of both spleen and lymph nodes was moderate whereas in B-cell compartments it was intense. A rapid growth of the LCM between 8 and 12 weeks p.i., 15-20 times greater than for the first 8 weeks, was associated with depletion of lymphocytes in thymus dependent area (TDA) of both spleen and lymph nodes, gross expansion of the red pulp with extramedullary haemopoiesis, partial atrophy of spleen follicles, but not those of the lymph node, and involution of the thymus. At 12 weeks p.i. the TDA had mainly plasma cells and histiocytes among occasional lymphocytes and blast cells; germinal centre activity and plasmacytosis persisted in B-cell areas. Morphological aspects of these changes are discussed in relation to the invasiveness and proliferation of the LCM during the course of infection.