In clinical routine use, fibrinogen is measured by clotting-time methods, or by clot turbidity in photometric prothrombin time determination. For calibration of these assays measurement of total thrombin-clottable protein has been recommended. We have now developed a microfiltration assay for total thrombin-clottable protein. Plasma samples were mixed with thrombin in a 96-well microfiltration device. After clot formation, the fluid was extracted by vacuum suction, and fibrin adherent to the filter membranes washed with buffer. Membrane segments with adherent fibrin were recovered from the 96-well manifold with a punch and transferred to tubes containing denaturing buffer solution. After dissolution of fibrin, protein concentration was determined by optical absorption at 280 nm. The microfiltration assay displayed a high correlation with the total clottable protein method (R = 0.95), and fibrinogen antigen (r = 0.96). Correlation with clotting time assays, and PT-derived fibrinogen in 150 clinical plasma samples was in the range of r = 0.84 to r = 0.97. Intraassay and day-to-day variability of the assay was comparable to the conventional total clottable fibrinogen assay. The novel microfiltration assay appears to be well suited for measurement of large series of samples for calibration, screening purposes, and clinical trials.