We published in the journal a new four-color flow cytometric method to help diagnose, stage, monitor, and screen autologous transplant products for children with neuroblastoma (1). The “four-color” monoclonal antibody assay involved a ganglioside GD2/fluorescein isothiocyanate (FITC), TAPA CD81/phycoerythrin, leukocytic CD45/peridinin chlorophyll protein, and neural cell adhesion molecule CD56/allophycocyanin cocktail. At that time, the only two sources available of any direct conjugate of ganglioside GD2 were the FITC products from Seikagaku America (Falmouth, MA, USA; clone GMR7, product no. 870633) and United States Biological (Swampscott, MA, USA; clone 8.S.077, product no. G2005-15). Both products were satisfactory from our experience, of the immunoglobulin M class, and actually the same clone. However, after publishing our results, the GD2-FITC product from Seikagaku was no longer commercially available and the United States Biological product failed our quality control test against the SK-N-DZ human neuroblastoma cell line (CRL-2149, American Type Culture Collection, Manassas, VA, USA). We had several e-mail inquires from around the world and informal inquires at the 2003 annual Clinical Cytometry Society Meeting in Arlington, Virginia by other flow cytometry laboratories that were trying to duplicate, unsuccessfully, our results. They cited an availability and performance problem with the GD2-FITC. We were unsuccessful in convincing Seikagaku to produce the product, but United States Biological was willing to bring the FITC conjugation of the GD2 monoclonal antibody “in-house” from its previously subcontracted “outside” source. Our quality control evaluation of the new product involved the dilution of 4 μl of stock product to 40 μl of buffer (phosphate buffered saline with 0.1% sodium azide) and adding 1, 2.5, 5, 10, and 20 μl of this “working dilution” to 1 million SK-N-DZ neuroblastoma cells in our four-color cocktail (United States Biological; ganglioside GD2, product no. G2005-15, lot no. L4101322). The 20 μl of working dilution (representing 2 μl of “neat” product) appeared optimal and similar to our previously published results in 2002 by producing a signal from the second to third decade on a four-decade scale as shown in Figure 1 (FACSCailibur, Becton Dickinson, San Jose, CA, USA). Forward versus side scatter light dot plot of the SK-N-DZ cell line (upper left), CD81/phycoerythrin (PE) versus CD45/peridinin chlorophyll protein (PerCP; upper right), CD56/allophycocyanin (APC) versus CD45-PerCP (lower left), and ganglioside GD2/fluorescein isothiocyanate (FITC) versus CD45-PerCP (lower right). We hope this technical note documents the importance of quality control in the clinical flow cytometry laboratory, the importance of working with our vendors, and encourage other clinical flow cytometry laboratories to evaluate this four-color assay for children with neuroblastoma. Michael J. Warzynski*, John L. Roys*, John W. Peterson , Humberto DeLa Vega , * Flow Cytometry LabSpectrum HealthGrand Rapids, Michigan, Technical Support United States BiologicalSwampscott, Massachusetts.