We studied the effects of cytokines such as IL-1 alpha, IL-6 and TNF alpha, and lipopolysaccharide (LPS) on cultured human monocytes. Increased fibronectin (FN) production, as a indicator of the activation of monocytes, was observed with any cytokines or LPS stimulation those were added in the culture medium. These increased FN production by cytokines showed a dose dependent fashion, and 4 hours culture with these cytokines was enough to obtain the amount of FN measured with radioimmunoassay. The combination of sub-optimal dose of cytokines (IL-1 alpha + IL-6, IL-1 alpha, IL-6 + TNF alpha), that could not induce substantial amount of FN with any single cytokine, also could induce FN by cultured monocytes. The specificity of cytokines for the FN production by cultured monocytes was confirmed by the neutralization using monoclonal antibodies specific for each monokines. Northern blot analysis with cDNA specific for FN confirmed the expression of FN mRNA in cultured monocytes stimulated with cytokines or LPS. Cytokine network plays an essential role for immune and inflammatory reaction, and FN has been shown to induce monokines through VLA-5 receptor on cultured monocytes. Our data suggests that monocytes may not always require high concentration of cytokines for the activation of monocytes in vitro, and that the synergistic action of low concentration of cytokines for the activation was enough for the progression of immune or inflammatory reaction.