Abstract Quantitative measurement of CD34 cells has become the gold standard for determining the sufficiency of peripheral blood progenitor cell (PBPC) collections to reconstitute patients after high-dose therapy. Flow cytometry is the accepted method for quantifying CD34 cells. However, there are concerns about the accuracy and reproducibility of this method. The IMAGN 2000 STELLer assay offers an alternative approach, with minimal sample manipulation and operator involvement. For this study we compared our flow results with STELLer on 50 mobilized PB samples obtained immediately prior to apheresis and the 50 matched PBPC harvests. Calibration and STELLer assays were run according to manufacturer's instructions. Flow staining and analyses consisted of two-color staining with CD14-FITC and CD34-PE reagents, acquisition of 100,000 events on a FACSCAN, and analysis using Winlist. Linear regression analyses indicated a strong correspondence between STELLer and flow results. On PBPC r2 = 0.88 for products ranging from 13-6000 CD34 cells/μl, for PB r2 = 0.75 (range 2–180 CD34/μl), and for all samples r2 = 0.92. As a general finding, STELLer counts were 10–20% lower than flow values. However, at lower cell numbers (counts < 10 CD34/μl) there was a tendency to report slightly higher results. We conclude that the STELLer assay can provide a highly accurate enumeration of CD34 cells in PB and PBPC samples, highly comparable with that obtained by flow cytometry.