Effects of plasma fraction I (‘fibrinogen’) on post-tonsillectomy bleeding were compared with surgical treatment and infusions of saline on randomized groups of adult patients. Surgical revision of the bleeding site arrested bleeding in every patient (13 cases), whereas infusion of 2 g of plasma fraction 1 effectively controlled bleeding in 32 out of 42 patients without any additional surgical treatment. In all of the patients receiving plasma fraction 1 without permanent arrest of bleeding the additional surgical revision was successful. Infusion of saline alone failed to influence post-tonsillectomy bleeding (6 patients). The difference in the effects of plasma fraction I and saline is statistically significant. Hemostatic and clinical evaluation of the patients with post-tonsillectomy bleeding did not demonstrate any apparent causes which would explain a bleeding tendency in comparison with a control group undergoing tonsillectomy without abnormal hemorrhage. In conclusion, infusion of 2 g of plasma fraction I is a partially effective treatment of post-tonsillectomy hemorrhage, but the mechanisms of action of this treatment remain unkn