Antigenic differences between the influenza C strains C/1233/47 (Taylor), C/Paris/1/67 and strains isolated in Johannesburg 1967 have been reported (Pereira, 1969). Our experiments performed with chicken immune sera did not differentiate clearly between the strains C/Paris/1/67 and C/Jhb/4/67, however, antigenic differences between the strains C/1233/47 and the strains isolated in 1967 could be confirmed. These antigenic differences were found to be of approximately the same degree as those observed between the strains influenza B/Berlin/55 and influenza B/Belfast/61. A serological survey showed that the prevalence of such influenza B strains was clearly delineated in a change of the predominant antibody related to age. In contrast, antibody patterns to the three influenza C strains were similar in all age groupds, most antibody carriers being detected with the C/1233 strain, and revealed to evidence for antigenic drift. Antibody distribution in sera obtained from paediatric hospitals of two cities was nearly the same with the highest prevalence reached in children aged 6-7 years. Low titered non-specific inhibitors of haemagglutination to the strains isolated in 1967 were present in some animal and human sera which could be completely adsorbed with kaolin.