Due to their dependence on negatively charged carbohydrate moieties (sialic acids) and their consequent inability to detect all glycoforms of the CD34 antigen (whose fine structure varies on different glycoforms of the CD34 molecule), class I antibodies fail to detect CD34 on some leukemias and leukemia-derived cell lines (2, 17, 28). Their somewhat lower avidity and general inability to retain reactivity after conjugation with the negatively charged fluorochrome, FTTC, further reduce their use for enumeration of CD34+ cells in PBSC samples and in protocols involving immunoselection of CD34+ cells.