Gangliosides were purified from the cells of two patients with hairy cell leukemia and one patient with chronic lymphocytic leukemia. Quantification of these compounds showed that these cells contain only 5-15% of the amount of lipid-bound sialic acid (gangliosides) per cell as normal lymphocytes. Structural characterization by gas-liquid chromatography, glycosidase treatment and high-performance liquid chromatography demonstrated that the major gangliosides of these leukemia cells were of the lactosyl type. Hairy cells contained monosialyllactosylceramide (II3NeuAc-LacCer), whereas chronic lymphocytic leukemia cells contained both monosialyl and disialyl lactosylceramide [II3(NeuAc)2-LacCer]. Chronic lymphocytic leukemia cell contained lesser amounts of three other gangliosides of the neolacto or lacto series as determined by endo-beta-galactosidase treatment. None of these leukemia cells contained detectable quantities of NeuAc-LcOse3Cer, a ganglioside found in normal leukocytes.