Ristocetin, at relatively low concentrations (1.0 mg/ml-1.5 mg/ml), can selectively precipitate fibrin monomers and fibrin degradation products (fdp) from plasma without effect on fibrinogen or fibrinogen degradation products (FDP). 125I-labeled fibrin monomers and fibrin degradation products were precipitated by ristocetin when their plasma concentrations were greater than 0.25 microgram/ml and 50 microgram/ml, respectively. In order to obtain a visible precipitated, 2 microgram/ml of fibrin monomers of 50 to 100 microgram/ml of fibrin degradation products were necessary. These effects were optimally observed under the following conditions: (1) temperature, 20 C to 37 C; (2) pH, 7.0 to 7.5; and (3) incubation time, 15 to 60 minutes. Late-fibrin degradation products are approximately eight times less sensitive to ristocetin-induced precipitation than early-fibrin degradation products. Plasma medium is essential for the differentiation of fibrin monomers and fibrin degradation products from fibrinogen and fibrinogen degradation products by ristocetin. These results suggest that the specific detection of fibrin monomers and fibrin degradation products in plasma may be easily performed by ristocetin.