This paper summarizes the available information on the relationship of two envelope antigens (haemagglutinin and neuraminidase) of influenzaviruses isolated from different hosts. The relationship of the haemagglutinin antigens was based on the results of haemagglutination inhibition tests with postinfection sera and that of the neuraminidase antigens on the results of neuraminidase inhibition and gel precipitation tests with hyperimmune and monospecific sera. On the basis of the antigenic specificity of the haemagglutinin, the influenzaviruses of human origin are divided into several subtypes (H0, H1, H2); viruses of equine origin could be divided into two subtypes (Heq1, Heq2). Porcine influenza strains are regarded as belonging to a single subtype, all of them being related to the prototype swine influenzavirus A (swine/Iowa/15/30). Within the avian influenzaviruses, 6 antigenic subtypes were described in earlier studies. Antigenic relationships between the haemagglutinin of strains from different hosts were infrequent but were demonstrated and confirmed between human A/Hong Kong/68 and equine viruses and between A/Hong Kong/68 and swine/Taiwan/69. The swine/Taiwan/69 virus also shared the related neuraminidase with A/Hong Kong/68 virus, and represents the only isolation from nonhuman sources of an influenzavirus identical with a human pandemic strain. The studies on the antigenic specificity of the neuraminidases demonstrated 8 antigenic varieties of neuraminidase among avian influenzaviruses and also that the neuraminidase grouping did not correspond with the antigenic grouping with regard to haemagglutinin. The relationships between human and nonhuman influenzaviruses are emphasized because of their significance to studies on the origin of influenza pandemics in man.