Peripheral blood samples from 115 consecutive patients and bone marrow samples from 9 healthy donors were studied for percentages of CD34-expressing cells, quantitative expression of various CD34 epitopes as defined by fluorescence mean channel, and mutual inhibition of the different CD34 monoclonal antibodies detecting the various CD34 epitopes. The study focused only on samples from patients with presumably elevated numbers of CD34-expressing cells, due to the nature of the disease. Samples from patients with chronic myeloproliferative syndromes, acute leukemias from nonhematological cancer patients during mobilization with filgrastim, and normal bone marrow samples were studied. Elevated numbers of CD34-expressing cells (> 0.04% of all nucleated cells) were detected in 111 of 124 patients. An almost identical expression of CD34 epitopes as detected by phycoerythrin-conjugated monoclonal antibodies QBEND-10, 8G12, and ICH3 were detected, whereas expression of the IMMU409 epitope was detected in only a few samples (19 of 111). Reactivity of IMMU-133 was almost identical to that of QBEND-10. Reactivity of BIRMA-K3, the only CD34 monoclonal used in this study, but not described in previous workshops, was almost identical to that of 8G12. From studies on mutual inhibition of binding, two families of CD34 epitopes are defined. The first family is comprised of QBEND-10, IMMU-133, My-10, and ICH-3, and the second one is comprised of only 8G12 and BIRMA-K3.