The neutrophil respiratory burst is essential for the host's ability to kill ingested microorganisms. Several flow cytometric assays have recently been developed to measure this process. In this study a whole blood flow cytometry assay using dihydrorhodamine 123 (DHR) as a substrate was compared with the quantitative nitrobluetetrazolium (NBT) test, an accepted measure of the earliest events in the respiratory burst. Because whole blood is used, the new assay is quicker and simpler than existing flow cytometry assays. Specimens as small as 0.1 ml can be used which makes the assay ideal for use in neonates and young children. It is concluded that the whole blood DHR assay is an accurate and sensitive measure of the respiratory burst.