Peripheral blood cells from 9 E- and 6 D+ SIg- ALL patients and from 11 CML patients in TdT+ blastic crisis were subjected to discontinuous albumin gradient separation according to Dicke's method, and TdT assayed in the six fractions recovered therefrom. The major results were: (i) in E- ALL and in CML-BC, TdT+ cells could be recovered either within a narrow range of specific densities or were spread over most of the gradient, which might suggest differences in cell maturation among the patients; (ii) in some instances, most leukemic blasts were found in TdT-negative or faintly positive fractions; (iii) in most, but not all, E- All and BC, the majority of TdT+ cells was found in low density fractions; (iv) all E+ ALL had high density TdT+ cells. Cell Fractions of most patients were also examined at the electron microscope, and correlations between morphology and marker characterization were tentatively drawn.