burstpromoting activity bpa without erythropoietin ep activity in sera of aplastic anemia patients fractionated by chromatofocusing column a chromatofocusing column was used to separate burstpromoting activity bpa and erythropoietin ep activity in sera from patients with aplastic anemia and interestingly a fraction containing a considerable level of bpa without ep activity was found at around ph 50 another fraction at lower ph contained both activities this is the first report of the existence of a fraction with bpa but without ep activity in sera from aplasia patients in trying to measure bpa in sera and urine a new assay procedure was used involving counting the number of erythroid bursts in normal human marrow cell cultures initiated with test samples to which 12 units of standard ep were added 56 days later separation of bpa from ep activity in patients sera was also attempted by sephadex g100 but this was unsuccessful due to the presence of inhibitors to burst formation also in a urine separation bpa and ep activities were eluted in the same fraction sephadex g100 chromatography gave results which seemed to indicate separation of bpa from ep activity however this may have been due to the presence of inhibitors this idea was supported by the fact that the peaks of both activities coincided when a urine preparation containing no inhibitors was tested