Control of neoplastic proliferation reflects in part monocyte/macrophage destruction of target cells--destruction that evidently requires cell-cell interaction. We herein show it to involve the natural plasma opsonin, fibronectin. With two cultured human tumor lines--Malme melanoma and CAK-I renal carcinoma cells--addition of fibronectin, purified to homogeneity, enhances macrophage-mediated cytotoxicity 2--4-fold (p less than 0.01). Both fresh human monocytes or the U-937-cultured macrophage line become more lethal to tumor cells with added fibronectin. The fibronectin-enhanced monocyte and U-937 tumoricidal activity occurred in a dose-dependent fashion. Specificity of fibronectin's action was validated by use of affinity-purified rabbit antifibronectin antibody, which completely abated its enhancement of tumoricidal activity. Enhancement of tumoricidal activity did not occur when monocytes or U-937 were exposed to fibronectin-coated plates. However, the addition of soluble fibronectin to fibronectin-coated plates was then capable of enhancing cytocidal activity. These studies demonstrate that human fibronectin is capable of increasing both fresh and cultured human monocyte tumor-directed cytotoxicity. Fibronectin appears to be a potentially important circulating molecule that may favorably influence human monocyte tumor cell cytotoxicity.