Flow cytometric analyses with monoclonal antibodies were so far restricted to leukocyte populations, which had been separated from erythrocytes before staining and/or analysis. Instead, whole blood staining methods allow for a rapid and accurate determination of cellular subpopulations in non-separated biological samples. This is not only time saving but reduces also the probability of an unintended loss of distinct cellular populations due to e.g. commonly used differential centrifugation procedures. With the NM-LYSE reagent flow cytometric analysis of whole blood has become as easy and accurate as the analysis of separated cell populations.