The accumulation of mononuclear phagocytes at sites of chronic inflammation is dependent on an increase in the rate of extravasation of blood-borne monocytes through the vascular endothelium into the connective tissue. Once the monocytes have emigrated into the connective tissue, they may differentiate into tissue macrophages, presumably following interactions with extracellular matrix proteins. To study these processes, we tested the effects of cytokines and phorbol esters on the adhesion of U937 cells, a human monocyte-like cell line, to cultured endothelial cells (EC) and to matrix proteins. In the absence of cytokines, very few of the U937 cells adhered to EC (5% or less in most experiments). When EC were pretreated for optimal periods of time (4-8 hr) with recombinant interleukin-1 alpha (IL-1 alpha), IL-1 beta, tumor necrosis factor-alpha (TNF alpha), or lymphotoxin (LT; also known as TNF-beta), 35-85% of the U937 cells were able to bind. Interferon-gamma (IFN-gamma) and interleukin-2 (IL-2) did not stimulate U937-EC binding, even though IFN-gamma was shown to increase EC adhesiveness for T lymphocytes. Phorbol esters also greatly stimulated U937-EC adhesion but, in this case, the increase was due to an action on the U937 cells. A monoclonal antibody (MAb), 60.3, against the CD11/CD18 family of leukocyte adhesion molecules partially inhibited the adhesion of untreated and phorbol ester-treated U937 cells to noncytokine-treated EC. However, that MAb had no effect on U937 cell binding to TNF-alpha-treated EC. Thus U937 cells use both CD11/CD18-dependent and -independent mechanisms to adhere to EC. In the absence of stimulating agents, only a small proportion of the U937 cells (2-20%) adhered to fibronectin (FN), and almost none bound to either laminin (LN) or gelatin (denatured type I collagen). In the presence of phorbol esters, a much larger proportion of the U937 cells adhered to FN, with only slight increases in the proportion of cells which bound to LN or gelatin. Additional adhesion assays performed in the presence of a pentapeptide containing the amino acid sequence arg-gly-asp (RGD), which is part of one of the cell-binding domains of FN, demonstrated that the RGD-containing peptide almost totally blocked the phorbol ester-induced adhesion of U937 cells to FN. In contrast, the peptide had no inhibitory effect on the phorbol ester-induced binding of U937 cells to EC.