A rapid and sensitive method for determination of plasma or serum opsonic fibronectin was developed and evaluated. This is a kinetic immunoturbidimetric assay based on the measurement of an antigen-antibody reaction. Comparison with electroimmunoassay was conducted in septic surgical and trauma patients with opsonic fibronectin deficiency before and after infusion of fibronectin-rich plasma cryoprecipitate. In contrast to the 24 to 48 hr needed for obtaining quantitative determinations by electroimmunoassay, the immunoturbidimetric assay can be accomplished within 10 min and requires only 5 microliters of serum or plasma. The concentration of opsonic fibronectin in plasma is greater than in serum. Plasma anticoagulated with EDTA and supplemented with aprotinin (Trasylol) to inhibit proteolytic activity is the procedure of choice for preparation of samples prior to analysis. The rapid immunoturbidimetric assay has clinical value as a rapid, sensitive, noninvasive means of assessing opsonic fibronectin and indirectly evaluating RES function. Its diagnostic as well as prognostic value with respect to various diseases remains to be carefully studied.