Of 1200 human sera tested by enzyme-linked immunosorbent assay (ELISA), 53% contained antibodies to human coronavirus (HCV) 229E and 88% to HCV OC43. The sera were from persons aged 13 months to 80 years, both males and females, and were collected in four different regions in 1986. The percentage of positives increased with increasing age and tended to vary according to the geographic area. Additional paired human sera from 218 patients with acute respiratory diseases (ARD) were collected between October 1986 and June 1987 in Prague. Significant antibody rises to HCV strain 229E were detected in 7 (3.2%) patients 9 months to 17 years old, to HCV strain OC43 in 4 (1.8%) patients under 2 years of age.