The distribution of leucocyte subpopulations in platelet concentrates (PC),derived from pre-storage filtered platelet-rich plasma (PRP), the cell suspension obtained by reverse filter washing and the post-filtered PC, were monitored by immunophenotyping analysis using CD3, CD20 and CD33. Leucocyte activation analysis with the CD11b marker revealed that this molecule is up regulated in neutrophils taken from the filter. This, together with the loss of cell viability during the enrichment process, suggests that contact with the filter matrix and processing and storage of samples containing leucocytes may lead to activation and loss of leucocyte viability. These changes were found to be more pronounced in less stable myeloid cells and account for the differences reported among various authors which in some cases related to operational conditions such as the enrichment process used and the length of time between filtration and analysis of samples. Finally, statistical analysis of the results obtained by immunophenotypic studies indicate that post-filter samples (S) contain significantly higher numbers of CD33+ myeloid cells when compared to (PF) the pre-filter samples (65.03% +/- 12.6 and 24.56% +/- 14.73, p < 0.0000), with a de crease in T cells (50.72% +/- 14.80 in PF and 24.05 +/- 9.48 in the cell suspension (S), p < 0.0007) and B cells (14.96 +/- 9.31 in PF and 9.9 +/- 5.22 in S, p < 0.201). A new strategy for assessing the influence of the filtration process on residual leucocyte activation and viability is described. This has direct relevance to collection, processing, storage and quality monitoring of PC. (C) 2003 Elsevier Science Ltd. All rights reserved.