Passengers injured in a bus collision sued Greyhound (D1), Schlagenhauf (D), the bus driver, and the owners of the tractor-trailer (D2) with which the bus collided. D2 claimed that the accident was due to D's negligence and moved for a physical and mental examination of him. D1 cross-claimed against the contract carrier (D3) and D2. D3 filed an answer to this cross-claim, stating that the collision was caused by D's negligence. D3 petitioned for a mental and physical examination of D under Fed. R. Civ. P. 35; the District Court granted the petition and ordered D to submit to nine different examinations. D applied for a writ of mandamus for the mental and physical examinations to be set aside. The Court of Appeals denied mandamus and the Supreme Court granted certiorari.