The proinflammatory cytokine interleukin-1beta (IL-1beta) is a secreted protein that lacks a signal peptide and does not follow currently known pathways of secretion. Its efficient release from activated immune cells requires a secondary stimulus such as extracellular ATP acting on P2X(7) receptors. We show that human THP-1 monocytes shed microvesicles from their plasma membrane within 2-5 s of activation of P2X(7) receptors. Two minutes after such stimulation, the released microvesicles contained bioactive IL-1beta, which only later appeared in the vesicle-free supernatant. We conclude that microvesicle shedding is a major secretory pathway for rapid IL-1beta release from activated monocytes and may represent a more general mechanism for secretion of similar leaderless secretory proteins.