A method for carrying out antibody absorption studies for antigenic analysis of group B arthropod-borne (arbor) viruses is described and examples of homologous and heterologous absorption curves are presented. Evidence that antigenic structure can be a stable property was obtained with three strains of West Nile virus isolated from different hosts in different countries over a period of years. Comparative studies with viruses of the Japanese B-St. Louis-West Nile subgroup indicate that each virus contains a completely specific antigen as well as one or more cross-reactive components. Strains of yellow fever virus isolated in America were shown to lack an antigen present in strains of African origin although no differences were found between isolates from the same geographical area. The attenuated 17 D vaccine strain of yellow fever was found to have acquired an additional antigen not present in the unadapted parent or in other strains tested. However, alteration in pathogenicity for man was not found to be necessarily attended by any antigenic modification, as shown by the antigenic identity of the French neurotropic vaccine strain with its pantropic parent.