In the present study, we demonstrate that the infection of human peripheral blood mononuclear cells (PBMC) with influenza A virus caused changes in intracellular fluorescein fluorescence polarization (IFFP) which, as previously described, reflect alterations in the polymerization of the cytoskeleton. Kinetic measurements revealed two cycles of an approximate 10% decrease in IFFP within 3.5 and 5 h after infection. Infection with influenza A virus also altered the response of PBMC to phytohaemagglutinin (PHA), which was manifested as changes of 5.3 and 4% in IFFP at 1 and 2 h after infection, respectively. The changes in IFFP correlated with DNA synthesis measured 72 h after exposure to PHA. These results show the ability of IFFP measurements to identify early intracellular metabolic events induced in virus-infected cells.