In the human lung carcinoma cell line A549, Taxol (20 nm)causes a decreased electrophoretic mobility of the 66-kDa Shc isoform(p66shc), beginning 4 h after drug exposure, and reaching a maximum at 9–18 h. No shift was observed for the 52- and 46-kDa isoforms of Shc. The electrophoretic mobility shift of p66shc caused by Taxol is not the result of tyrosine phosphorylation, and there is no indication of a Shc/Grb2 complex in Taxol-treated A549 cells. This modification is blocked by the serine/threonine protein phosphatase 2A. In vivo ³²P-labeling and subsequent phosphoamino acid analysis of p66shc indicated that both the original and the shifted p66shc were predominantly serine phosphorylated. Cyanogen bromide digestion of p66shc produced a phosphorylated fragment with an apparent molecular weight of ∼7.9 kDa from the untreated cells and two phosphorylated fragments, of ∼7.9 and ∼9.6 kDa, from the Taxol-treated cells. The domain of Taxol-induced serine phosphorylation is thought to be in the cyanogen bromide fragment containing residues 2–65. The Taxol-induced electrophoretic mobility shift of p66shc was inhibited by the protein synthesis inhibitor, cycloheximide, but not by the mitogen-activated and extracellular signal-regulated protein kinase kinase (MEK)inhibitor, PD98059. This mobility shift did not occur in Taxol-resistant A549-T12 cells treated with 20 nm Taxol. In addition to Taxol, other microtubule-interacting drugs caused a decreased electrophoretic mobility of p66shc. This Taxol-mediated serine phosphorylation seen in p66shc may result from a MEK-independent signaling pathway that is activated in cells that have a prolonged or abnormal mitotic phase of the cell cycle and may play a role in signaling events that lead to cell death.