Although bone marrow aspiration is the most commonly obtained human marrow sample type, it yields liquid samples that contain a varying degree of blood contamination. Trephine biopsies, on the other hand, are solid marrow cores and are, therefore, much less likely to contain contaminating peripheral blood. In this study, we utilised a technique to extract viable cells from solid trephine biopsy specimens, by means of mechanical and enzymatic digestion, allowing cytometric comparison of cells in these biopsies and simultaneously obtained liquid aspirate samples. Having established that the digestion procedure itself was not causing any significant alteration in the immunophenotypic composition of the marrow samples, our data show that trephine biopsies were enriched for CD8(+) T cells, with concomitant decrease in the CD4(+) subset, compared to paired aspirates. Furthermore, T cells, NK cells and T cells expressing NK cell receptor (NKR) molecules were all significantly more likely to express both early (CD69) and late (HLA-DR) markers of activation. Bone marrow aspirates do not, therefore, provide truly representative data on the phenotypic composition of bone marrow, and the effect of peripheral blood contamination in aspirates should be taken into account when comparisons are being made between the bone marrow and other human issues or, perhaps more so, between human and murine marrow.