Specimens of blood collected in Tecumseh, Michigan over a four-year period were studied for rise in antibody titer against coronavirus OC43. Peaks of infection were found in the winter and spring of 1966, 1968, and 1969; at other times, infections occurred sporadically. All age groups were involved, especially the very young. Rises in titer by CF and by HAI tests frequently did not occur together in the same individual. Agreement between the two tests was better in 1966 and 1969 than in the other years. A portion of the paired specimens showing rises in CF and/ or HAI titer was tested by neutralization. Rises in neutralizing antibody were usually found in pairs collected in 1966 and 1969 but not in those collected in 1967 and 1968. The infecting viruses in 1966 and 1969 thus appeared more closely related to OC43 than did those in 1967 and 1968.