Peel, core and dice the apples into inch pieces. Cut the grapes in half. Coarsely chop the walnuts.
Combine the diced celery, apples, grapes, and walnuts together with the lemon juice. Fold the yogurt into the mixture, chill and serve.
Nutritional information per serving: 220 calories, 9 g fat, 0.5 g saturated fat, 0 mg cholesterol, 35 mg sodium, 27 g carbohydrates, 4 g fiber, 10 g protein.