Most studies of the induction of cytokine synthesis in monocytes have employed an exogenous triggering agent such as lipopolysaccharide. However, in nonseptic inflammatory responses (e.g. rheumatoid arthritis) monocyte activation occurs as a result of T cell-generated signals. In previous reports, we and others have demonstrated that contact-dependent T cell-generated signals are capable of contributing to macrophage activation. We have shown that plasma membranes from anti-CD3 activated purified peripheral CD4+ T cells (TmA) but not from resting CD4+ cells (TmR) induce monocytes to synthesize interleukin (IL)-1 in the absence of co-stimulatory cytokines. Studies to determine the expression kinetics of the molecule(s) unique to activated CD4+ T cells which interact with monocytes to induce IL-1 revealed that optimal expression occurred at 6 h post activation. This matched the previously reported kinetics of expression of CD40 ligand (CD40L) on activated peripheral T cells, implicating the CD40-CD40L interaction as a candidate for the initiator of the IL-1 signaling event. The ability of TmA to induce IL-1 synthesis in resting monocytes could be markedly reduced by addition of a monoclonal anti-CD40L antibody, 5c8. In addition, a monoclonal anti-CD40 IgM (BL-C4) proved dramatic in its ability to induce resting monocytes to synthesize IL-1. In summary, these results demonstrate that the CD40-CD40L interaction provides a critical component of CD4+ T cell contact-dependent activation of monocyte IL-1 synthesis.