INTRODUCTION:: The function of CD34, a transmembrane sialomucin expressed by human haematopoietic progenitor cells, is poorly understood. Its structure suggests it may act as a cell adhesion and signalling molecule. MATERIALS AND METHODS:: KGIa cells and primary CD34-positive marrow cells were tested for their ability to aggregate in the presence of the anti-CD34 antibody QBEND10; CFU-GM colonies were grown using standard methods and tested for their content of colony-forming cells by replating; `haematons' were isolated from marrow by filtration; the phosphorylation of CD34 was investigated by immunoprecipitation and Western blotting DISCUSSION:: CD34-positive cells in human bone marrow, like KG1a cells, aggregate when incubated with QBEND10. Staining aggregates with anti-CD34-FITC revealed that aggregation involved co-localisation of CD34 at intercellular binding sites. We examined myeloid colonies (CFU-GM) grown from normal human bone marrow cells, and multicellular aggregates (`haematons') separated from freshly aspirated marrow by filtration, and found CD34-positive cells bound together with co-localisation of the CD34 at the binding sites. This finding shows that CD34-positive cell-cell adhesion occurs physiologically in vitro and in vivo. QBEND10-induced aggregation of KG1a and CD34-positive cells was enhanced by staurosporine (a protein kinase C inhibitor) and inhibited by genistein (a protein tyrosine kinase inhibitor). Moreover, aggregated cells had increased phosphorylation of tyrosine on CD34 and translocation of protein kinase C (PKC) to the cytoplasm, compared with non-aggregated cells. We used the ability of primary colonies to produce secondary colonies on replating as a functional parameter and found that the replating ability of the colonies was increased by treatment with genistein (P=0.003). In addition, the ability of individual samples of primary CD34-positive cells to undergo QBEND10-induced aggregation and the ability of CD34-positive... [ABSTRACT FROM AUTHOR]