1. A stem cell suppressing factor has been found in the serum of a patient with Hodgkin's disease, a thymic tumour and pure red‐cell aplasia. This factor disppeared when active erythropoiesis returned following radiotherapy to the tumour. 2. The suppressing activity was assayed in mice by measuring the capacity of their bone marrow to restore erythropoiesis in heavily irradiated recipients. 3. Failure to demonstrate a raised erythropoietin level, as well as the finding that IgA and IgM immunoglobulins were reduced, suggested that autoimmune processes might be involved. However, it appeared unlikely that the factor was an autoantibody since suppressing activity was destroyed by storage at −20° C. 4. The sera of nine other patients, one with a thymoma associated with erythroid and megakaryocyte aplasia, three with anterior mediastinal tumours and five with secondary marrow aplasia, were assayed. Three of these showed mild suppressing activity, but this could have been due to non‐specific causes, the nature of which is discussed.