The breadth of the antibody response to antigenic subunits and whole virus vaccines was compared on sera against eight influenza A strains. The spectrum of cross-reactions for the two kinds of vaccine was statistically indistinguishable, both in antihemagglutinin and in neutralization tests. A strong negative correlation was found between the capacity of an antigen to give broad immunologic coverage and its proneness to being inhibited by heterologous sera. A model is proposed to account for the asymmetry of serologic crossing, and its theoretically and practical implications are discussed.