United Airlines (UA) flight 2428 received a traffic alert and collision avoidance system (TCAS) resolution advisory (RA) while descending to flight level FL310 when enroute to San Francisco International Airport (SFO), San Francisco, California. Two passengers were seriously injured, and two cabin crewmembers sustained minor injuries as a result of the aircraft response to the TCAS alert. The flight was a regularly scheduled domestic passenger flight from Newark Liberty International Airport (EWR), Newark, New Jersey to SFO. UA2428 was instructed by air traffic control to descend and maintain FL310 for their arrival into SFO. About 500 ft above level off at FL310, the crew received a TCAS traffic alert “traffic, traffic” for an aircraft crossing 1,500 feet below to which, the first officer, as pilot flying, reduced vertical speed on the mode control panel in response. A TCAS RA then immediately annunciated for the same traffic. The pilot flying responded by disengaging the autopilot and auto throttle and pitched the aircraft up following the pitch guidance on the primary flight display. Flight data show pitch increased by about 3° over 1 second and the aircraft descent arrested. Vertical acceleration ranged from 2.3 to 0.6 (g) over 2 seconds during the maneuver. The seatbelt sign had been turned on in the cabin shortly prior to the TCAS annunciation, however passengers still remained in the lavatories at the time of the maneuver. Two flight attendants were in the forward galley cleaning in preparation for the initial descent announcement. These two flight attendants fell to the floor during the maneuver and sustained minor injuries. There were two passengers in the aft lavatories at the time of the maneuver. One passenger flew upwards and landed forward fracturing the L2 spinal vertebrae. The other passenger was exiting the lavatory when he flew upwards and landed on his leg resulting in a fractured ankle. Following the event, the aircraft landed uneventfully. Paramedics met the aircraft at the gate and transported the passengers to the hospital for treatment.