We comparatively assessed CD34Ψ cell quantification by two of the recently available single platform assays, the IMAGN2000 STELLer (Immucor, Lisbon, Portugal) microvolume fluorimetry and the Pro-COUNT (BD-ENZIfarma, Lisbon, Portugal) flow cytometry, with our ''in-house'' dual-platform flow cytometric assay. The performance of the methods was evaluated by linearity and reproducibility tests. The linearity study, over a range of 0-1,200 CD34Ψ cell/Β΅l, gave a good linear relationship for the three methods, with R 2 G 0.99. Precision tested at three different concentrations gave coefficients of variation ranging from 3.6-26.4% for the STELLer, 2.4-13.8% for the ProCOUNT, and 3.2-6.4% for flow cytometry. CD34Ψ cells were quantified in umbilical cord blood (UCB), UCB enriched-leukocyte buffy-coat (BC), mobilized peripheral blood (PB) and mobilized peripheral blood progenitor cells (PBPC) collected by leucapheresis, from a total of 72 samples. Flow cytometric results showed good linear correlation to the absolute counts obtained by the STELLer and ProCOUNT for all samples (R G 0.90 for all methods), with no differences when compared by paired tests (P G 0.05). Linear correlations between methods were also found when individually looking at the different cell sources: UCB or PB, BC, and PBPC, with low, intermediate and high CD34Ψ cell concentrations, respectively. Furthermore, with the exception of a significant difference between the ProCOUNTand STELLer results for UCB (P F 0.05), no other difference between methods was found for each of the individual populations (P G 0.05). To our knowledge, this is the first report in which the results are presented and analyzed according to each source of CD34Ψ cells. Our results show that the STELLer and the ProCOUNT are equally efficient for the dual-platform flow cytometric assay in CD34Ψ cell quantification.