T-DM1 disrupts the platelet microtubule cytoskeleton but does not affect spreading. Resting and spread (glass-activated) platelet cytoskeletal structure was examined in washed platelets from at least 3 different human donors before and after incubation with a vehicle control or 100 μg/mL of T-DM1 or 5B6-DM1 (trastuzumab specificity control) for 4 hours. Platelets were permeabilized and probed for β1-tubulin (green) and phalloidin (red). Resting platelets showed aberrant morphologies (ie, diffuse tubulin staining, tennis racket forms, less uniformity in platelet diameter, and smaller platelets) in the presence of T-DM1 and 5B6-DM1, but normal spreading on glass.