Clinical trials of the pancreatic ribonuclease (RNAse) in treatment of patients with tick-borne encephalitis were carried out. Two hundred forty-six patients were treated with RNAse and 261 were given antiencephalitic gamma globulin. Administration of 30 mg of RNAse every four hours over a period of five to six days provided stable high concentrations of the enzyme in blood that exceeded normal values by two to three times. An increase of RNAse activity in cerebrospinal fluid also took place. The comparison of RNAse and gamma globulin demonstrated a considerable advantage of the former: under RNAse treatment the temperature normalized on the average of twice as quickly, and a rapid regression occurred in meningeal symptoms and cerebrospinal fluid pleocytosis. In patients treated with RNAse, no adverse reactions or aftereffects were observed. On the grounds of the reported results, RNAse was accepted officially as a new, highly efficient antiviral drug for the treatment of tick-borne encephalitis.