Circulating erythroid progenitors from 14 patients with acute lymphoblastic leukemia (ALL) and from 8 healthy subjects were studied in culture to determine the frequency and size of CFU-E- and BFU-E-derived colonies. Cells were cultured in a plasma clot system, and hemoglobinized colonies identified by diaminobenzidine reaction. The numbers of CFU-E and BFU-E per milliliter of peripheral blood were greatly increased in 10 patients when compared to controls. In 13 patients, the size distribution of BFU-E-derived colonies, analyzed by counting the number of subunits in each colony, was also found to differ significantly from controls, with a large excess of small colonies and a low percentage or a total lack of large colonies. This abnormal BFU-E size distribution was partially corrected, in the 5 patients tested, by the addition to the culture medium of 10% phytohemagglutinin-leukocyte-conditioned medium (PHA-LCM). Bone marrow crowding out of the normal progenitors, as well as disturbances in the cellular interactions involved in their normal development, most likely explain these results and these factors could be implicated in the frequent pancytopenia of ALL.