After parallel analysis of a large number of samples of normal blood, mobilized peripheral blood, cord blood, and normal marrow, as well as of CD34+ cell lines that fail to express some class I CD34 epitopes, it is our experience that PE conjugates of QB End 10 (class II, Immunotech or Southern Biotechnology Associates), 8G12 (HPCA2, class III, Becton Dickinson), and the recently available 581 (17) class III, Immunotech) all work equally well in the current ISHAGE protocol (see Fig. 3). If the basic protocol is modified to incorporate a third conjugated antibody in a three-color analysis (26, 27) and FITC conjugates of CD34 antibodies have to be used, class III reagents, such as 8G12 and 581, can be recommended. However, for the reasons outlined, the use of FITC conjugated class II CD34 antibodies is not recommended.