To determine whether tyrosine phosphatases become activated upon infection with influenza virus a phosphatase assay using the Tyrosine Phosphatase Assay System (Promega) was performed. A459 cells were infected for 10 h (MOI = 5) or left uninfected. Cells were harvested in assay buffer (100 mM tris-HCl pH 5.2, 100 mM CaCl 2 , 100 mM MgCl 2 , 0.02% b-mercapto ethanol), cracked by a single freeze/thaw step at 280uC and disrupted by ultrasonic pulsing. Lysates were precleared from cell debris and residual free phosphates according to the manufacturer's instruction. Tyrosine phosphatase activity was measured by enzymatic release of free phosphate of two given pseudosubstrates (phosphorylated peptides representing target sequences for the most common tyrosine phosphatases). Quantification was performed in comparison to a given standard according to the manufacturer's instruction.