This paper presents the characterization of a sugar-specific receptor on the surface of human circulating poly-morphonuclear cells. With the help of fluorescent neo-glycoproteins and flow cytometry, a receptor was identified as being specific for α-L-rhamnosyl residues. The number of receptors was 55 000/cell and their affinity reached 2 × 108 I mol−1. This number changed as a function of the biological state of the cells. Indeed, receptor expression was modulated by the presence of other cells. T cells and B cells increased the number of receptors on the granulocyte surface. Expression of the α-L-rhamnose-specific lectin was dependent on lymphocyte derived soluble factor(s), which induce(s) growth and differentiation of polymorphonuclear phagocytes. Granulocyte/ macrophage colony-stimulating factor (GM-CSF) specifically produced a significant increase in the number of receptors for α-L-rhamnose (2–10-fold/cell). This modulation was independent of protein kinase C activators such as phorbol ester, which produced no effect on α-L-rhamnose receptor expression. These findings demonstrate that GM-CSF may stimulate post differentiation functions and properties of mature granulocytes.