Morphological alterations in the spleen, liver, lymph nodes, thymus, bone marrow and blood were studied in DBA and Balb/C mice treated with Corynebacterium parvum. The spleen lymph nodes and liver increased in size and weight, whereas thymus showed decrease in weight compared to the control group. A marked, early proliferation of monocytes in the bone marrow preceded infiltrations by macrophages in lymph nodes, spleen and liver. These infiltrations were closely related to the vascular structures, became nodular and decreased around day 12 after the injection. A rapid increase in the number of monocytes and transformed lymphocytes of the peripheral blood were noted almost immediately after the administration of the vaccine. No relationship between the degree and duration of the histological changes, strain of mice, and number of inoculations was found. It is suggested that the increase in size and weight of the lymphoid organs and the liver is primarily caused by invasion of macrophages, perhaps secondary to a preceding proliferation of monocytes in the bone marrow.