The amount of CD34+ cells is important to predict the quality of stem cell transplants and ensure safe engraftment after high-dose chemotherapy. Further, daily controls of the patients' peripheral blood CD34+ cell counts are performed to optimize peripheral blood stem cell collection, especially in patients with low CD34+ cell numbers. Therefore, the use of reliable reference samples is mandatory for quality assurance, both in terms of patient safety and reproducibility of results from different laboratories. We report our first experience with CD-Chex CD34, containing stabilized placental cord blood cells in preservative medium with defined concentrations of CD34+ cells. Analysis was performed according to standard operating procedures. Three lots were tested sequentially on a day per day use. The expected CD34+ values were 28.6-, 36.7- and 28.1 microl(-1), respectively, and the mean measured values were 27.4 +/- 2.75 microl(-1) (n = 25, range 21 - 33 microl(-1), coefficient of variation [CV] 10.0%), 29.9 +/- 2.39 microl(-1) (n = 17, range 26 - 35 microl(-1), CV 8.0%), and 27.2 +/- 2.24 micro1(-1) (n = 18, range 24 - 34 microl(-1), CV 8.2%). Serial dilution (1:2 to 1:10) with normal peripheral blood or PBS w/o Ca++/Mg++ gave adequate results. We conclude that the control samples used in this setting are reliable and thus helpful to increase the accuracy in the analysis of CD34+ cells.