Sema7A is a recently described member of the semaphorin family that is associated with the cell surface via a glycophosphatidylinositol linkage. This study examined the mRNA expression and biological properties of this protein. Although the expression of Sema7A was demonstrated in lymphoid and myeloid cells, no stimulation of cytokine production or proliferation was evident in B or T cells. In contrast, Sema7A is an extremely potent monocyte activator, stimulating chemotaxis at 0.1 pm and inflammatory cytokine production (interleukin-1 (IL-1β), tumour necrosis factor-α (TNF-α), IL-6 and IL-8) and superoxide release at 1–10 pm. Sema7A is less effective at stimulating neutrophils. Sema7A also significantly increases granulocyte-macrophage colony-stimulating factor (GM-CSF) production from monocytes but has no consistent effect on IL-10, IL-12 or IL-18. Sema7A can also induce monocytes toward a dendritic cell morphology. Sema7A is expressed in monocytes and probably released through proteolysis and acts as a very potent autocrine activator of these cells.