Human serum contains an activity which enhances the release of colony stimulating and burst promoting activity (CSA and BPA). It is low in the majority of normal sera and was found elevated in patients with aplastic anaemia. The patient's 'response' to autologous serum releaser activity was measured by comparing CSA and BPA release by patient cells in percentage of release by normal cells. This 'response' was negatively correlated with serum releaser activity (P = 0.0035 for CSA-release, P less than 0.0001 for BPA-release), i.e. releaser activity was high, when factor release was low. This inverse relationship between releaser activity and the patient's response was also observed in four patients with pancytopenia of causes other than aplastic anaemia. We conclude that elevated serum releaser activity reflects a repair mechanisms which operates when CSA- and BPA-production is inadequate. Thus, releaser activity is likely to be a haemopoietic regulator.