We prospectively studied serum antiviral antibody response in adult patients with febrile upper respiratory tract infections. Paired sera were obtained in 158 patients from January 1982 to December 1983. Eighteen patients (11.4 percent) showed a significant antibody rise in the convalescent phase of the disease compared to the acute phase. Significant antibody rise was observed against all viruses examined: influenza viruses type A and B, adenovirus type 3, respiratory syncytial virus, and parainfluenza viruses type 1, 2 and 3. Two patients responded simultaneously to two viruses. Serum interferon was detected in acute phase of the disease in 4 of 8 patients examined in whom viral infections were proved. Detected interferon was alpha type.