A human cell line (U937) can be stimulated to produce a soluble factor (MCF) by either lectin-activated T lymphocytes or their soluble products. In prior studies, we showed that MCF produced by peripheral blood mononuclear cells can increase production of collagenase and prostaglandin E2 by adherent synovial cells obtained from enzymatically dispersed rheumatoid synovium. We show here that peripheral blood T lymphocytes or cloned human T lymphocyte lines are capable of inducing MCF production by the monocyte-like U937 cells. MCF can be demonstrated in the supernatant fluid from cocultures of U937 cells and T lymphocytes that have been stimulated with phytohemagglutinin or concanavalin A for 24-48 hr. In addition, the supernatant fluid from 24-hr lectin-stimulated T lymphocytes can be transferred onto the U937 cells and subsequently, MCF activity can be recovered from the U937 culture medium. The activity of the soluble T cell product on the U937 cells is both time- and dose-dependent. A human cell line capable of MCF production in continuous culture has not been previously available. The use of a monocyte-like cell line (U937) and cloned T lymphocytes now makes it possible to demonstrate the role of discrete cell populations in the production of MCF and other mediators.