The peripheral blood of acute myeloblastic leukemia (AML) patients often contains large numbers of two distinct cell populations, both capable of forming colonies in culture under similar conditions. The first population consists of the precursors of blast cells and has specificity for AML; the second population consists of T-lymphocyte precursors, also found in normal blood. The two progenitor populations can be separated by exploiting the capacity of T-lymphocyte (but not blasts) progenitors to form rosettes with sheep erythrocytes (E rosettes). After E-rosette formation, T-lymphocyte precursors can be removed by centrifugation on Ficoll-Hypaque. Such separation has a number of consequences: (1) Blast progenitors can be detected where unseparated mononuclear preparations have yielded either no colonies or only T-lymphocyte colonies (20 of 21 patients). (2) The stimulator requirements of the blast progenitors change, indicating that cell-cell interactions may take place between blast and T-lymphocyte progenitors. (3) It is feasible to characterize blast and T-lymphocyte precursors independently, even though they may coexist in peripheral blood. This may be important if progenitor properties are attributes contributing to the variance in outcome in AML.