Microparticles are small vesicles released from the plasma membrane of various cell types independently of apoptosis or cell death, are transferred between cells, and carry membrane proteins from one cell to another. We have studied the mechanism of uptake of microparticles by monocytes and B cells. The transfer of microparticles to B cells was almost completely dependent on complement. Incubation of microparticles with serum resulted in opsonization of microparticles with the complement cleavage product iC3b. The subsequent transfer to B cells was mediated by the complement receptor CR2. The interaction between iC3b-opsonized microparticles and B cells reduced the activation of B cells as measured by expression of MHC class II, CD86 and CD25. In contrast, transfer of microparticles to monocytes was only partially complement dependent, but involved calcium and annexin V, and was found to change the cytokine profile of monocytes towards a reduced release of the pro-inflammatory cytokines GM-CSF and TNF-a and an increased release of the anti-inflammatory cytokine IL-10. These data show that microparticles are taken up by B cells and monocytes by different mechanisms and modulate the activation of monocytes and B cells towards an anti-inflammatory phenotype. Microparticles might be involved in counterbalancing pro-inflammatory signals arising from tissue injury or inflammation.