Colony formation in vitro by marrow cells from patients with untreated acute myelogenous leukemia (AML) and from patients in AML relapse is infrequent using the standard Robinson assay. A newly developed culture system has been described in which marrow from AML patients in these disease stages form leukemic cell colonies. In this in vitro system, phytohaemagglutinin is the essential stimulator for colony formation. The leukemic origin of the colonies has been proven by ultrastructural morphology and cytogenetics. It appears that colony formation by leukemic cells in this system is predominantly independent from the leukocyte factor which is the main stimulator in the Robinson assay for growing colonies of marrow cells from haematologically normal individuals.