Sequential serum specimens were obtained every four months during 1975-1979 from 44 children and adults of 10 Seattle families. The 419 specimens were tested for antibody to human coronaviruses OC43 and 229E by enzyme-linked immunosorbent assay (ELISA). Antibody titers were found to increase with age, and titers as well as frequency of rises were greater for OC43 than for 229E virus in all age groups. Significant antibody rises were most frequent in specimens bracketing the winter interval, but some also occurred in the spring-summer and summer-fall intervals. Concurrent significant antibody rises to OC43 virus in different members of the same family were observed in 15 instances, to 229E virus in seven instances, and to OC43 virus in some members and 229E virus in others in eight instances. Significant antibody rises to OC43 or 229E virus indicating reinfections were frequently observed throughout the three-year period but were always separated by at least two four-month intervals. Concurrent significant antibody rises to both 229E and OC43 viruses were seen only in three persons. Finally, the frequency of significant antibody rises in children, about one per person-year, was almost three times higher than in adults.