The blood concentration of the high-molecular-weight glycoprotein fibronectin showed great promise as a marker for cancer and certain other disease states. However, it now appears that plasma fibronectin levels are influenced by many factors (e.g., age, sex, nutritional state, trauma, shock, inflammation, certain drugs), sometimes giving conflicting responses. Furthermore, unrecognized pitfalls exist in fibronectin determinations with respect to blood sampling methods, plasma preparation, plasma storage, and re-use and assay methods. It is concluded that the plasma fibronectin level, as frequently reported, has little value as a marker for cancer and other disease states.