Human Coronavirus OC43 from suckling mouse brain (SMB) suspension has been adapted to grow in human embryonic lung fibroblast cell cultures. Complement-fixing and hemagglutinating antigens were prepared from infected cell cultures and used in parallel with the same antigens from SMB for a study of the prevalence of OC43 antibody in Northern Italy as well as for the serodiagnosis of recent OC43 infections and reinfections. Results show (i) a higher sensitivity of the hemagglutination-inhibition as compared to the complement fixation test both for antibody detection in seroepidemiological studies and serodiagnosis of acute infections, (ii) a high prevalence of OC43 antibody in Northern Italy, so that only exceptionally are people older than 12 years seronegative, (iii) a high rate of OC43 reinfections during the winter-spring season 1976/77 in a population sample with preexisting antibody (iv) a good quality of reagents prepared from cell cultures, and hence free of contaminating mouse viruses, such as mouse hepatitis virus.