Mouse bone marrow as a source of plaque-forming cells (PFC) was studied by single and multiple intravenous injections of sheep erythrocytes (SRBC). In the primary response a great number of PFC appeared in the spleen while only a few showed up in the marrow. In the secondary response there is a clear PFC-response in both the spleen and the bone marrow. The spleen contains the majority of PFC until about 9 days after the second injection. In the course of the reaction, however, the number of PFC in the bone marrow rises to a level which surpasses the level in the spleen. IgM-PFC as well as IgG-PFC and IgA-PFC could be demonstrated in the marrow. In the lymph nodes and Peyer's patches the number of PFC did not increase above the normal background level at any time after the primary or secondary immunization. In order to test whether the bone marrow PFC-response is caused by a migration of PFC from the spleen or by development in situ, mice were splenectomized shortly before the second injection of SRBC. It could be shown that splenectomy does not prevent the bone marrow PFC-response. Because no activity in the other lymphoid organs was observed, it is concluded that the PFC activity of the marrow is caused by development in situ.