A study is described in which filamentous forms of influenza virus were observed budding from host cell surfaces. Cell cultures infected with influenza virus were stained by indirect immunofluorescence using an antiserum to purified haemagglutinin. Filaments greater than 100 micrometers in length, with several branch points along their length were observed; the number and length of filaments varied according to the virus strain and the time after infection. Examination of infected cells by electron microscopy confirmed the presence of branched structures with an ultrastructure typical of filamentous forms of influenza virus. The immunofluorescence technique was quicker than thin section electron microscopy and was a more sensitive procedure for the detection of filamentous forms of influenza virus than electron microscopy using negative stain. It also enabled the antigenic composition of the filaments to be observed.