SUMMARY In 15 to 60% sucrose density gradients the distribution of standard pre- parations of influenza viruses measured by haemagglutination or infectivity was always bimodal. Approximately I7% of virus particles sedimented in a diffuse leading peak (fraction I), while the rest was confined to a second homo- geneous peak (fraction 2). Virus particles comprising fraction i were less dense (1"25o g./cm?) than those of fraction 2 (1.257 g./cm?). RNA labelled with ~zp was extracted by phenol + sodium dodecyl sulphate from four strains of influenza A virus (PR 8, BEL, A2/SINGAPORE/I (57 and fowl plague) and from one strain of influenza B virus (LEE). The sedimentation characteristics of each preparation of RNA on sucrose density gradients varied under different conditions of ionic strength. In the presence of o.I M-NaCl+o.ooi M-EDTA, RNA from fraction 2 could be resolved into at least two species. The predominant species had a sedimentation coefficient of 18 to 21 S and (with the exception of the A2 RNA) about IO to 15 % of the total RNA sedimented as a diffuse band with a sedimentation coefficient of 32 to 42 S. The BEL and LEE strains contained a third species of RNA, with a sedimentation coefficient of 7 to 9 S. In 0"005 M-EDTA without NaCI the RNA from fraction 2 0'RS) sedimented principally as a I4 S component, and the 32 to 42 S component was not found. Fraction I virus particles also contained several RNA components when analysed under conditions of high ionic strength. A 7 to 9 S RNA species was found in amounts equal to, or greater than, the 18 to 21 S RNA component, together with a small amount of' 38' S RNA. In the presence of 0"005 M-EDTA the 38 S component was not found, but three components with sedimentation coefficients of I6 S, 9 S and 5 S were present in approximately equivalent amounts. From these findings it is concluded that the RNA of influenza viruses consists largely of x 8 S mole- cules, under defined conditions of ionic strength, and that the 32 to 42 S com- ponent is an artifact caused by aggregation. It is further concluded that sucrose gradients containing o.i M-NaC1 are unsatisfactory for studying the sedi- mentation characteristics of influenza RNA.