Isolated progenitors of defined type are useful for definitive studies of commitment, growth factor specificity, and identification of viral target cells. Blast cell colonies from cultures of spleen cells obtained from mice treated with 5-fluorouracil have proven to be a useful source of pluripotent progenitors. These colonies also contain committed progenitors of some lineages such as the macrophage lineage; however, the incidence of progenitors for pure erythroid colonies is low in this population. We report here the development of an alternative blast cell colony assay using fetal liver cells that is based on the greater sensitivity of early progenitors to interleukin 3. Replating studies demonstrated a variety of types of secondary colonies, including macrophage, neutrophil, neutrophil/macrophage, megakaryocyte, erythroid, mast cell, and mixed. The incidence of pure erythroid and mast cell colony-forming cells was substantially higher in fetal liver blast cell colonies than in colonies cultured from adult spleen.