One hundred ten to 120-kDa fragments of fibronectin (FNf), generated by proteases released in the course of tissue injury and inflammation, stimulate monocytes to produce proinflammatory cytokines, promote mononuclear leukocytes (MNL) transendothelial migration, up-regulate monocyte CD11b and CD86 expression, and induce monocyte-derived dendritic cell differentiation. To investigate whether the proinflammatory consequences of FNf are offset by responses that can suppress proliferation of activated T lymphocytes, we investigated the effect of FNf-treated MNL on autologous T lymphocytes induced to proliferate by substrate-immobilized anti-CD3. FNf-stimulated MNL suppressed anti-CD3-induced T cell proliferation through both contact-dependent and contact-independent mechanisms. Contact-independent suppression was mediated, at least in part, by IL-10 and TGF-beta released by the FNf-stimulated MNL. After 24-48 h exposure to FNf, activated T cells and monocytes formed clusters displaying CD25, CD14, CD3, and CD4 that were not dissociable by chelation of divalent cations. Killing monocytes with l-leucine methyl ester abolished these T cell-monocyte clusters and the ability of the FNf-stimulated MNL to suppress anti-CD3 induced T cell proliferation. Thus, in addition to activating MNL and causing them to migrate to sites of injury, FNf appears to induce suppressor monocytes.