A simple, rapid, and accurate semiautomated micromethod for the determination of plasma fibrinogen is described. One tenth milliliter of plasma is diluted in 1.9 ml. barbital buffer (pH 7.5). The clotting time is determined by an electronic tinier after addition of 0.1 ml. thrombin (100 U. per ml.) to 0.1 ml. diluted plasma. The fibrinogen level is read from a calibration curve. Abnormal levels are confirmed by a chemical method. Discrepancies between results obtained by the two methods may be a clue to structural abnormalities of fibrinogen or the presence of fibrin split products.