The object of this study was to determine whether antigenic groupings exist among influenza B viruses. Altogether, 22 influenza type B strains isolated during the years 1940-68 were examined by reciprocal haemagglutination-inhibition, strain-specific complement-fixation, and serum neutralization tests with sera produced in ferrets and guinea-pigs. It was found that the strain-specific complement-fixation test was superior for separating influenza B viruses into groups whereas the haemagglutination-inhibition and serum neutralization tests were better for demonstrating similarities. The results obtained with these three immunological techniques confirmed that antigenic variation exists among influenza B viruses, although it is not as clearcut as among influenza A viruses.The results were subjected to numerical taxonomic analysis. Dendrograms and minimum-spanning trees were constructed, using methods based on cluster analysis of similarity coefficients. Four main groups of influenza B viruses were established, although they were all interlinked. The results of this study do not justify the separation of influenza B viruses into subtypes similar to those of influenza A viruses.