It has been suggested that CD44 mediates adhesive interactions between hematopoietic progenitor cells and the stromal microenvironment. Ligands of CD44 include several extracellular matrix components, such as hyaluronic acid and fibronectin. Antibodies against CD44 have been shown to induce homotypic T cell aggregation, and to stimulate T and natural killer cell activity. We hypothesized that CD44 could similarly amplify interactions between blast-colony-forming cells and bone marrow stromal cells (BMSCs). Indeed, we have previously found that the anti-CD44 antibody NKI-P2 enhanced VLA-4-dependent interactions. Here, we studied an additional panel of nineteen anti-CD44 antibodies from the 5th Workshop on Leukocyte Differentiation antigens, to find out whether amplification was associated with a particular CD44 epitope. None of these antibodies showed inhibitory activity, whereas nine significantly increased the number of blast colonies more than 2-fold. Seven of these recognized epitope 1, and two epitope 2. More than 4-fold enhancement was only observed with epitope 1 antibodies: 4.C3 (4.4-fold), 212.3 (6.3-fold), L178 (9.1-fold), and NIH44-1 (9.2-fold). Our data suggest that primarily epitope 1 is associated with enhancement of colony formation. Furthermore, the findings support a role for CD44 as an amplifier in progenitor-BMSC interactions.