The UK NEQAS Immune Monitoring Scheme (UK NEQAS) evaluates the performance of laboratories routinely performing T-lymphocyte subset analysis on HIV-infected individuals. The scheme originally issued fresh whole blood, but a significant problem was that of analyte stability, especially 36 h postphlebotomy. To circumvent this problem, we have developed a novel stabilisation procedure that ensures retention of leucocyte light scatter and immunological staining characteristics for up to 300 days. In addition, the stabilised whole blood preparation is fully compatible with flow cytometer technology, incorporating either whole blood lysis or “no wash, no lyse” techniques. The ranges of interlaboratory coefficient of variation for the stabilised material are now tighter than those previously obtained with fresh whole blood. Development of this novel material has enabled overseas laboratories to participate in the UK NEQAS Immune Monitoring Scheme and could, in the future, lead to the production of reference and/or calibration reagents for leucocyte immunophenotyping. © 1996 Wiley-Liss, Inc.