A serological study of coronaviruses with the OC 43 agent (HI-test) was undertaken in 1000 sera from the population of Hamburg. 32,7% of the sera showed HI=titers between 1 : 8 and 1 : 128. The geometric mean titer (GMT) for the whole group was 1 : 16. The frequency of seropositive reactions was highest in 244 individuals 15 to 24 years old (52,8% positive reactions, GMT 1 : 19) whilst in the older age groups (25-59 years and over 60 years old) the precentage of positive reactors was markedly lower (14,4% and 16,8%, respectively, GMT 1 : 12). Among 131 twelve to 17 years old youngsters, a seroconversion was demonstrated in paired sera in 6,8% with a 4- to 8-fold rise in HI-titer within 8 weeks.