Burst-promoting activity (BPA) was measured in the sera from 31 children with aplastic anaemia (AA). BPA levels were elevated in most of the children with AA (65.2%), the mean value (137.7 +/- 18.4%) being significantly higher than that in normal children (69.6 +/- 9.4%), in children in the recovery period and in children with non-aplastic anaemia. There was a negative relationship between the BPA level in children with AA and the peripheral haemoglobin concentration. The BPA level was higher in those whose duration of illness was shorter than 1 year. In three cases of AA caused by chloramphenicol and benzene hexachloride and one case of congenital pure red cell AA, the BPA level was not elevated. Eleven patients received fetal liver cell suspensions intravenously (FLI). After FLI the BPA level in their sera was significantly reduced. According to these results, it appears that the elevation of BPA level is a special phenomenon of AA. The measurement of BPA in serum is helpful for differentiation between AA and other kinds of anaemia. The elevation of the BPA level in serum is a biological compensation for the haematopoietic disorder, and the measurement of BPA in the serum of patients with AA may be helpful in evaluating the haematopoietic condition.