In a 5- to 6-quart heavy pot combine sugar, grapefruit juice, and lemon juice. Bring mixture to a full rolling boil, stirring constantly. Stir in pectin. Return to a full rolling boil, stirring constantly. Boil hard for 1 minute, stirring constantly. Remove from heat. Quickly skim off foam with a metal spoon.

Ladle hot jelly into hot sterilized half-pint canning jars, leaving a 1/4-inch headspace. Wipe jar rims; adjust lids and screw bands.

Process filled jars in a boiling-water canner for 5 minutes (start timing when water returns to boiling). Remove jars from canner; cool on wire racks. Makes 5 half-pints.