[en] Our results, using radiation-induced bone marrow chimeras, demonstrate that the Ia antigen found in the brains of such animals is produced by cells having precursors in the bone marrow. These cells are not immediately blood borne since no IgM is detected in these brains. This rules out the obvious possibility of B-lymphocyte contamination as the source of Ia in the brain cell preparations. It thus appears that the central nervous system, like many other nonlymphoid organs, has a source of Ia-positive cells that are derived from bone marrow precursors