Close physical proximity between mast cells and T cells has been demonstrated in several T cell-mediated inflammatory processes. However, the way by which mast cells are activated in these T cell-mediated immune responses has not been fully elucidated. We previously identified and characterized a novel mast cell activation pathway initiated by physical contact with activated T cells and showed that this pathway is associated with degranulation and cytokine release. In this study, we provide evidence that mast cells may also be activated by microparticles released from activated T cells that are considered miniature versions of a cell. Microparticles were isolated from supernatants of activated T cells by Centricon filtration or by high-speed centrifugation and identified by electron microscopy, flow cytometry (Annexin stain), and expression of the integrin LFA-1. Stimulated T cells were found to generate microparticles that induce degranulation and cytokine (IL-8 and oncostatin M) release from human mast cells. Mast cell activation by T cell microparticles involved the MAPK signaling pathway. The results were similar when mast cells were stimulated by activated fixed T cells or by whole membranes of the latter. This suggests that microparticles carry mast cell-activating factors similar to cells from which they originate. By releasing microparticles, T cells might convey surface molecules similar to those involved in the activation of mast cells by cellular contact. By extension, microparticles might affect the activity of mast cells, which are usually not in direct contact with T cells at the inflammatory site.