In vitro colony formation in two different soft agar systems was studied with bone marrow from untreated patients suffering from acute myeloid leukemia (AML) and from rats in various stages of two different transplantable myeloid leukemias. In both the thin-agar-layer system, which uses a feeder layer of fetal fibroblasts, and the Robinson culture system, human AML marrow failed to produce colonies. A similar failure was observed with the BN rat leukemia. In contrast, the Shay rat leukemic marrow produced an abnormally large number of colonies in the later stages of the disease. Evidence was obtained that the colonies produced by the Shay leukemic marrow consisted of leukemic cells, and that the disappearance of colonies from human AML and from BN rat leukemic marrow is caused by the numerical disappearance of normal pluripotent hemopoietic stem cells from the marrow and by the inability of the clonogenic leukemic cells to produce colonies in vitro. The results indicate that the BN rat leukemia is a realistic animal model for human AML.