In vitro cell culture techniques to identify progenitor cells have been available for 20 years and depend on the ability of these cells to form colonies of fully differentiated cells in semisolid medium in response to specific growth factors. These colony assays have been used to investigate in vitro regulation and various biological characteristics of both multipotent and lineage-committed progenitor cells (see reviews 7 to 9). Shortly after the development of normal colony culture techniques these methods were applied to acute myeloblastic leukemias and culture conditions were defined that allowed growth ofcolonies ofleukemic cells.’#{176} 8