Masurel, Nic (Dept. of Virology, Medical Faculty Rotterdam, P.O. Box 1738, Rotterdam, The Netherlands), and William M. Marine. Recycling of Asian and Hong Kong influenza A virus hemagglutinins in man. Am J Epidemiol 97: 44–49, 1973.—Pre-Asian/57 and post-Hong Kong/ 68 epidemic sera were studied for antibodies to A/Japan/305/57 (H2N2), A/Hong Kong/1/68 (H3N2), and A/Equine/63 (Heq2Neq2) viruses by the hemagglutination-inhibition test. The relationship of antibodies in pre-Asian/57 sera further established the hypothesis that a Hong Kong/ 68-like virus stimulated antibody cross-reacting asymmetrically with Hong Kong/68 and Equine/63 hemagglutinins. The lack of a predictable quantitative and qualitative relationship of Asian/57 and Hong Kong/68 antibodies in the pre-Asian/57 sera, together with the different timing of increase in prevalence to these two hemagglutinins, lead to a unified hypothesis for recycling of Asian/57-like and Hong Kong/68-like viruses. The positive relationship between Asian/57 and Hong Kong/68 antibodies in the pre-Asian/57 and post-Hong Kong/68 collection of sera is explained by the anamnestic stimulation of Asian/57 antibody. A hemagglutinin-mediated anamnestic response in Asian/57 antibody by Hong Kong/68 vaccine was confirmed in a group of young adults, using hemagglutinin-specific recomhinants. Although the data do not permit the exact timing of this recycling, we can be quite certain that influenza viruses with Asian/57-like and Hong Kong/68-like hemagglutinins occurred in the same sequence at the end of the 19th century as was seen in 1957 and 1968. The implications of this conclusion include the prediction that a Swine-like influenza A virus may recur in man by 1985–1990.