The antigenicity of the hemagglutinin (HA) of influenza A0 and A1 strains will be analyzed by means of monoclonal BALB/c antibodies produced in vitro in an adoptive transfer system (splenic fragment cultures). Such antibodies constitute homogeneous populations of antibody combining sites and fulfill therefore the ultimate criterion of monospecificity for individual antigenic determinants. These antibodies will be applied in a radioimmunoassay to the antigenic analysis of the HA of various laboratory-maintained A0 and A1 virus strains in order to characterize the antigenic relationship among H0 and H1 at the level of individual HA-determinants and to estimate the number of determinants that compose the overall antigenicity of the HA. In addition, the monoclonal antibodies will be used for the selection of mutant viruses in an attempt to define the antigenic variability of the HA in terms of the extent and the frequency of given antigenic changes in individual HA-determinants.