On July 16, 1993, about 0315 Atlantic standard time, a Boeing 767-322ER, N645UA, registered to the Wilmington Trust Company, operated by United Air Lines, Inc., under 14 CFR Part 121, flight No. 976, as a scheduled, international, passenger flight, experienced in-flight turbulence when the flight was about 60 miles north of the Isla de Margarita, Venezuela. Instrument meteorological conditions prevailed at the time and an IFR flight plan was filed. The airplane was not damaged and the captain, first officer, relief pilot, 6 flight attendants, and 152 passengers were not injured. Two flight attendants sustained minor injuries and one flight attendant sustained serious injuries. The flight originated from the Ezeiza Airport, Buenos Aires, Argentina, at 2159 local time. The captain stated that he was on his rest break with the relief pilot occupying his seat. During cruise at flight level 350, the flight encountered the top of cirrus clouds followed by two sharp jolts of turbulence that lasted about 15 seconds. The weather radar was on and no weather was depicted. The flight continued to the scheduled destination and after landing, the injured flight attendants were examined and treated for their injuries.