Peel and pit peaches. Pit plums. Cut fruits in small pieces or chop. Measure into large kettle. Add sugar and lemon (very thinly sliced) and stir to mix well.
Boil rapidly, stirring constantly, until jellying point is reached, or until thick. Remove from heat; skim and stir alternately for 5 minutes. Ladle into hot sterilized jars; seal.