The RNA of the human influenza virus Singapore (H2N2) strain has been labeled in vivo by phosphorus-32 and separated by polyacrylamide gel electrophoresis into eight segments, which were correlated to the corresponding gene functions and/or proteins. The base sequence homology between the individual genes (segments) of the H2N2 virus and those of different influenza A strains has been determined by molecular hybridization. Segments 1, 5, 7, and 8 of the Singapore strain exhibit a base sequence homology of almost 100% as compared to the FM1 strain (HlNl), while the homology between the other segments is significantly lower (24–76%). For the Singapore and Hong Kong (H3N2) strains all segments except that coding for the hemagglutinin (HA, 24%) exhibit a homology close to 100%. The ³²P-labeled segment 4 (HA-gene) of the avian influenza A strain duck Ukraine (Hav7Neg2) shows a homology of 92% to Hong Kong, while the homology of at least two other segments is significantly lower. These results are taken as an indication that the H2N2 subtype is derived from the HlN1 subtype by a recombination event retaining four H1N1 segments, while the other four segments are gained from another yet unknown strain. The H3N2 subtype is presumably derived from a H2N2 subtype, retaining seven segments of the H2N2 subtype, while the gene coding for the HA is obtained from the duck Ukraine or another highly related strain.