Influenza B viruses evolve more slowly than human influenza A, but no reasons for the difference have been established. We have analyzed sequence changes in the hemagglutinin and neuraminidase of influenza B viruses (and have determined four hemagglutinin sequences, of B/Bonn/43, B/USSR/100/83, B/Victoria/3/85, and B/Memphis/6/86) in relation to antigenic properties and compared these with similar analyses of variation in influenza A antigens. Independent of the slower rate of change in influenza B antigens, only approximately 30% of nucleotide changes in either the hemagglutinin or neuraminidase gene sequence result in amino acid changes in the protein, whereas in influenza A 50% of nucleotide changes result in altered amino acids. Thus, there is less selection for change, or less tolerance to change, in the influenza B antigens. This is similar to findings with influenza C and findings with influenza A viruses that replicate in lower animals and birds and is closer to the type of variation found in other RNA viruses. We propose that human influenza A is unique in that it is the only virus group in which antibody selection dominates evolutionary change.