A clonal method in cell culture is described that permits the quantitation of blast precursors in common (non-T, non-B) ALL; the method also yields information about progenitor properties, based on analysis of cells in colonies. The technique is identical to that used successfully for normal and malignant B-cell progenitors except that it requires culture at below O2 tension (5%-7%); mononuclear cells from blood or marrow are depleted of T cells and cultured with media conditioned by T cells in the presence of phytohemagglutinin and irradiated T cells. Cultures are incubated for 5-7 days in a moist atmosphere at 5% CO2 and 5%-7% O2. Colonies were obtained from marrow or blood of 16 of 18 ALL patients. Cells in colonies had the same characteristics (E-, slg-, cALL+ and clgM+ or clgM-) as the cells in the patient. By replating pooled colonies, self-renewal of progenitors was shown. The findings are considered in light of a model of leukemic blasts that depicts such populations as lineages maintained by progenitors that either renew themselves or give rise to blast cells with little or no proliferative capacity.