Male.mice were injected into their foot pads with sheep erythrocytes (SRBC) to form lymph follicles in the germinal centers in the popliteal lymph nodes. 4 weeks later, peritoneal macrophages labeled with carbon from syngeneic donors sensitized with SRBC or typhoid-paratyphoid bacilli (TAB) were separately injected into the foot pads as well. The popliteal lymph nodes were histologically examined at 6 h to 5 days after injection. Labeled macrophages appeared in the marginal sinus, migrated straight across the cortex from the marginal sinus to the lymph follicles and then entered the germinal centers. There was no difference in the mode of appearance, migration and localization of labeled macrophages in the regional lymph nodes between the mice given labeled macrophages from SRBC-sensitized donors and those given macrophages from TAB-sensitized donors. The entrance of lymph macrophages into the germinal centers of the regional lymph nodes would be immunologically nonspecific. After the injection of Pelikan ink into the foot pads, the macrophages which have taken up carbon in the peripheral tissue reached the regional lymph nodes via the afferent lymphatics and then entered the germinal centers, mainly through the medullary pole of the lymph follicles, after migrating along their immediate exterior from their marginal sinus to their medullary pole.