The effects of shear stress on the keratin intermediate filament (KIF) cytoskeleton of cultured human alveolar epithelial (A549) cells have been investigated. Under normal culture conditions, immunofluorescence revealed a delicate network of fine tonofibrils containing KIFs, together with many nonfilamentous, keratin-containing "particles," mostly containing either keratin 8 (K8) or 18 (K18), but not both. Triton X-100 extracted approximately 10% of the cellular keratin, and this was accompanied by a loss of the particles but not the KIFs. Shear stress dramatically reduced the soluble keratin component and transformed the fine bundles of KIFs into thicker, "wavy" tonofibrils. Both effects were accompanied by the disappearance of most keratin particles and by increased phosphorylation of K8 and K18 on serine residues 73 and 33, respectively. The particles that remained after shearing were phosphorylated and were closely associated with KIFs. We suggest that keratin particles constitute a reservoir of protein that can be recruited into KIFs under flow, creating a more robust cytoskeleton able to withstand shear forces more effectively.