Recent studies on sera from patients with aplastic anaemia show it to contain both stimulators and inhibitors of granulopoiesis. We have examined the serum of patients with aplastic anaemia in a short-term liquid culture system of human bone marrow which permits CFU-C proliferation. Increases in CFU-GM numbers over control values were observed with aplasia sera. All sera produced increased megakaryopoiesis, but no alteration in growth of the fibroblasts and macrophages of the adherent stromal layer. With one exception the sera had no significant direct colony stimulating activity when assayed in agar culture, but increased colony numbers were observed when the sera was incorporated into a sub-optimal feeder layer of peripheral blood leucocytes. When aplastic serum was added to adherent cell-depleted liquid cultures, the CFU-C stimulating property was lost, but increased megakaryopoiesis still occurred. These results indicate the presence of a direct-acting thrombopoietin and an indirect-acting stimulator of CFU-C proliferation in the serum of patients with aplastic anaemia, which may be related to the pathophysiology of the condition.