Summary The ability of human hematopoietic cell lines to form colonies in semisolid agar has been studied. Cloning efficiencies were as follows: 2 Burkitt lymphoma cell lines, over 70%, and a third, 18%; 2 myeloma cell lines, 40% and 11%, respectively; and 2 leukemia cell lines, 20% and 18%, respectively. In contrast, 6 cell lines derived from normal persons and 1 from a patient with Wegener's granulomatosis had cloning efficiencies below 4.5%. Doubling time and the presence or absence of the “leukovirus” (a herpes-type virus) did not affect cloning efficiencies. There was some evidence that abnormal chromosome constitution and the synthesis of an abnormal end product were correlated with a greater cloning efficiency. Cloning efficiency may be useful in distinguishing between “normal” and “abnormal” human hematopoietic cell lines.